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Fractal behaviour, i.e. scale invariance in spatio-temporal dynamics, has been found to describe and model many systems in nature, in particular fluid mechanics and geophysical related geometrical objects, like the convective boundary layer of cumulus cloud fields, topographic landscapes, solar granulation patterns, and observational astrophysical time series, like light curves of pulsating stars. The main interest in the study of fractal properties in such physical phenomena lies in the close relationships they have with chaotic and turbulent dynamic. In this work we introduce some statistical tools for fractal analysis of light curves: Rescaled Range Analysis (R/S), Multifractal Spectra Analysis, and Coarse Graining Spectral Analysis (CGSA), an FFT based algorithm, which can discriminate in a time series the stochastic fractal power spectra from the harmonic one. An interesting application of fractal analysis in asteroseismology concerns the joint use of all these tools in order to develop classification criteria and algorithms for ¿ Scuti pulsating stars. In fact from the fractal and multifractal fingerprints in the background noise of the light curves we could infer properties of different mechanisms of stellar dynamic, among them rotation, modes excitation and magnetic activity. © 2018 The Author(s). ; SdF, JPG and RG acknowledge funding support from Spanish public funds for research under projects ESP2015-65712-C5-3-R. JCS acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R, and from project RYC-2012-09913 under the 'Ramon y Cajal' program of the Spanish MINECO. SdF, JPG and RG acknowledge support from the 'Junta de Andalucia' regional government under project 2012-P12-TIC-2469. ; Peer Reviewed

Fractal fingerprints have been found recently in the light curves of several 5 Scuti stars observed by Convection Rotation and planetary Transits (CoRoT) satellite. This sole fact might pose a problem for the detection of pulsation frequencies using classical pre-whitening techniques, but it is also a potentially rich source for information about physical mechanisms associated with stellar variability. Assuming that a light curve is composed of a superposition of oscillation modes with a fractal background noise, in this work we applied the Coarse Graining Spectral Analysis (CGSA), a fast Fourier transform (FFT)-based algorithm, which can discriminate in a time series the stochastic fractal power spectra from the harmonic one. We have found that the fractal background component is determining the frequency content extracted using classical pre-whitening techniques in the light curves of 5 Scuti stars. This might be crucial to understand the amount of frequencies excited in these kinds of pulsating stars. Additionally, CGSA resulted to be relevant in order to extract the oscillation modes, this points to a new criterion to stop the pre-whitening cascade based on the percentage of fractal component in the residuals.© 2019 The Author(s). ; SdF, JP-G, JRR, ML-M, and RG acknowledge financial support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severn Ochoa' award for the Institute de Astrofisica de Andalucia (SEV-2017-0709). SdE, JP-G, JRR, ML-M, and RG acknowledge funding support from Spanish public funds for research under projects ESP2015-65712-C5-3-R. JCS and AGH acknowledge funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R. SdF, JP-G, and RG acknowledges support from the 'Junta de Andalucia' regional government under project 2012-P12-TIC-2469. JCS also acknowledges funding support from project RYC-2012-09913 under the 'Ramon y Cajal' program of the Spanish MINECO. ; Peer Reviewed

Open Access. Attribution 4.0 International (CC BY 4.0) ; This paper aims at being a provocative guide to the future of asteroseismology from the perspective of the analysis of time series, where the fundamentals of harmonic analysis are subjected to stress tests. In this context, we give an annotated summary of our research over the last decades on harmonic analysis of A-F stars. We discuss and explore the consequences of our findings, which may extend to any kind of pulsators. As well, we analyse the impact of this reconsideration on future asteroseismic studies, which would entail a paradigm shift. This includes a discussion on the presence of fractal behavior in delta Sct stars, and how this can be used to develop a stopping criterion of the pre-whitening process, as an alternative to SNR (or significance) criterion. Drilling a scientific paradigm has its natural resilience, hence the path described here is being arduous, although fruitful at the same time. ; SF, JP-G, JR, ML-M, and RG acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709) and Spanish public funds for research under projects ESP2015-65712C5-5-R. JS and AG acknowledge funding support from Spanish public funds for research under projects ESP2017-87676-C5-2R. JS also acknowledges funding support from project RYC2012-09913 under the Ramon y Cajal program of the Spanish MINECO. AG acknowledges support from Universidad de Granada under project E-FQM-041-UGR18 from the Programa Operativo FEDER 2014-2020 programme by Junta de Andalucia regional Government. ; Peer reviewed

Context. It is known that the observed distribution of frequencies in CoRoT and Kepler δ Scuti stars has no parallelism with any theoretical model. Pre-whitening is a widespread technique in the analysis of time series with gaps from pulsating stars located in the classical instability strip, such as δ Scuti stars. However, some studies have pointed out that this technique might introduce biases in the results of the frequency analysis. Aims. This work aims at studying the biases that can result from pre-whitening in asteroseismology. The results will depend on the intrinsic range and distribution of frequencies of the stars. The periodic nature of the gaps in CoRoT observations, only in the range of the pulsational frequency content of the δ Scuti stars, is shown to be crucial to determining their oscillation frequencies, the first step in performing asteroseismology of these objects. Hence, here we focus on the impact of pre-whitening on the asteroseismic characterization of δ Scuti stars. Methods. We select a sample of 15 δ Scuti stars observed by the CoRoT satellite, for which ultra-high-quality photometric data have been obtained by its seismic channel. In order to study the impact on the asteroseismic characterization of δ Scuti stars we perform the pre-whitening procedure on three datasets: gapped data, linearly interpolated data, and data with gaps interpolated using Autoregressive and Moving Average models (ARMA). Results. The different results obtained show that at least in some cases pre-whitening is not an efficient procedure for the deconvolution of the spectral window. Therefore, in order to reduce the effect of the spectral window to a minimum, in addition to performing a pre-whitening of the data, it is necessary to interpolate with an algorithm that is aimed to preserve the original frequency content.© ESO 2018. ; JPG acknowledge support from the "Junta de Andalucía" local government under project 2012-P12-TIC-2469. JCS and AGH acknowledge funding support from Spanish public funds for research under project ESP2015-65712-C5-5-R (MINECO/FEDER), and from project RYC-2012-09913 under the "Ramón y Cajal" programme of the Spanish MINECO. JCS also acknowledge support by the European FP7 project "SPACEINN" (FP7-SPACE-2011-1). AM acknowledges support by the Spanish MINECO National Space Program through project ESP2015-65712-C5-1-R and "Ramon y Cajal" program RYC-2012-09913. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 749962. ; Peer Reviewed

We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS 2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these and the singly periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor of 6 smaller than that in the B filter, which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 \mumag, respectively, in two of the best characterized non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. ; This work was supported by FCT - Fundacao para a Ciencia e a Tecnologia through national funds and by FEDER through COMPETE02020 - Programa Operacional Competitividade e Internacionalizacao by these grants: UID/FIS/04434/2019, PTDC/FIS-AST/30389/2017 & POCI-01-0145-FEDER-030389. MC is supported in the form of work contract funded by national funds through FCT. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant agreement no.: DNRF106). DLH and DWK acknowledge financial support from the Science and Technology Facilities Council (STFC) via grant ST/M000877/1. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. LFM acknowledges support from the UNAM by the way of DGAPA project PAPIIT IN100918. The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No670519: MAMSIE) and from the Fonds Wetenschappelijk Onderzoek -Vlaanderen (FWO) under the grant agreement G0H5416N (ERC Opvangproject). MS acknowledges the financial support of Postdoc@MUNI project CZ.02.2.69/0.0/0.0/16 027/0008360. EN acknowledges the Polish National Science Center grants no. 2014/13/B/ST9/00902. JCS acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R, and from project RYC-2012-09913 under the 'Ramon y Cajal' program of the Spanish Ministry of Science and Education. AGH acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R of the Spanish Ministry of Science and Education. AS, ZsB, and RSz acknowledge the financial support of the GINOP-2.3.2-15-2016-00003, K-115709, K-113117, K-119517, and PD-123910 grants of the Hungarian National Research, Development and Innovation Office (NKFIH), and the Lendulet Program of the Hungarian Academy of Sciences, project No. LP2018-7/2018. GH has been supported by the Polish NCN grant 2015/18/A/ST9/00578. MLM acknowledges funding support from Spanish public funds for research under project ESP2015-65712-C5-3-R. JPG acknowledges funding support from Spanish public funds for research under project ESP2017-87676-C5-5-R. MLM and JPG also acknowledges funding support from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Institute de Astrofisica de Andalucia (SEV-2017-0709). IS acknowledges funding support of NSF under projects DN 08-1/2016 and DN 18/13-12.12.2017. P. Kolaczek-Szymanski acknowledges support from the NCN grant no. 2016/21/B/ST9/01126. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. ; Peer Reviewed

We present the first asteroseismic results for δ Scuti and γ Doradus stars observed in Sectors 1 and 2 of the TESS mission. We utilize the 2-min cadence TESS data for a sample of 117 stars to classify their behaviour regarding variability and place them in the Hertzsprung-Russell diagram using Gaia DR2 data. Included within our sample are the eponymous members of two pulsator classes, γ Doradus and SX Phoenicis. Our sample of pulsating intermediate-mass stars observed by TESS also allows us to confront theoretical models of pulsation driving in the classical instability strip for the first time and show that mixing processes in the outer envelope play an important role. We derive an empirical estimate of 74 per cent for the relative amplitude suppression factor as a result of the redder TESS passband compared to the Kepler mission using a pulsating eclipsing binary system. Furthermore, our sample contains many high-frequency pulsators, allowing us to probe the frequency variability of hot young δ Scuti stars, which were lacking in the Kepler mission data set, and identify promising targets for future asteroseismic modelling. The TESS data also allow us to refine the stellar parameters of SX Phoenicis, which is believed to be a blue straggler. © 2019 The Author(s) ; We thank the referee for useful comments and discussions. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Funding for the TESS Asteroseismic Science Operations Centre is provided by the Danish National Re-search Foundation (Grant agreement no.: DNRF106), ESA PRODEX (PEA 4000119301), and Stellar Astrophysics Centre (SAC) at Aarhus University. We thank the TESS and TASC/TASOC teams for their support of this work. This research has made use of the SIMBAD data base, operated at CDS, Strasbourg, France. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-2655. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (Grant agreement no.: DNRF106). MC was supported by FCT -Fundacao para a Ciencia e a Tecnologia through national funds and by FEDER through COMPETE2020 -Programa Operacional Competitividade e Internacionalizacao by these grants: UID/FIS/04434/2019, PTDC/FIS-AST/30389/2017, and POCI-01-0145-FEDER-030389. MC is supported in the form of work contract funded by national funds through FCT (CEECIND/02619/2017). JDD acknowledges support from the Polish National Science Center (NCN), grant no. 2018/29/B/ST9/02803. AGH acknowledges funding support from Spanish public funds for research under projects ESP201787676-2-2 and ESP2015-65712-C5-5-R of the Spanish Ministry of Science and Education. FKA gratefully acknowledges funding through grant 2015/18/A/ST9/00578 of the Polish National Science Centre (NCN). JPe acknowledges funding support from the NSF REU program under grant number PHY-1359195. APi and KK acknowledge support provided by the Polish National Science Center (NCN) grant No. 2016/21/B/ST9/01126. This project has been supported by the Lendulet Program of the Hungarian Academy of Sciences, project No. LP2018-7/2018, and by the support provided from the National Research, Development and Innovation Fund of Hungary, financed under the K16 funding scheme, project No. 115709. JCS acknowledges funding support from Spanish public funds for research under projects ESP2017-87676-2-2 and ESP2015-65712-C5-5-R, and from project RYC2012-09913 under the 'Ramon y Cajal' program of the Spanish Ministry of Science and Education. The research leading to these results has (partially) received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement N.670519: MAMSIE), from the KULeuven Research Council (grant C16/18/005: PARADISE), from the Research Foundation Flanders (FWO) under grant agreement G0H5416N (ERC Runner Up Project), as well as from the BELgian federal Science Policy Office (BELSPO) through PRODEX grant PLATO. SBF acknowledges support by the Spanish State Research Agency (AEI) through project No. 'ESP2017-87676-C5-1-R' and No MDM-2017-0737 Unidad de Excelencia `Maria de Maeztu'-Centro de Astrobiolog ' ia (CSICINTA). ZsB acknowledges the support provided from the National Research, Development and Innovation Fund of Hungary, financed under the PD1717 funding scheme, project no. PD-123910. DLB acknowledges support from the Whitaker Foundation. SC gratefully acknowledges funding through grant 2015/18/A/ST9/00578 of the Polish National Science Centre (NCN). CCL gratefully acknowledges support from the Natural Sciences and Engineering Research Council of Canada. GMM acknowledges funding by the STFC consolidated grant ST/R000603/1. RMO, SC, and DR were supported in this work by the 'Programme National de Physique Stellaire' (PNPS) of CNRS/INSU co-funded by CEA and CNES. IS acknowledges the partial support of projects DN 08-1/2016 and DN 18/13-12.12.2017. PS acknowledges financial support by the Polish NCN grant 2015/18/A/ST9/00578. MS acknowledges the Postdoc@MUNI project CZ.02.2.69/0.0/0.0/16-027/0008360. JPG, JRR, and MLM acknowledge funding support from Spanish public funds for research under project ESP2017-87676-C5-5-R and from the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). JAE acknowledges STFC for funding support (reference ST/N504348/1). LFM acknowledges the financial support from the DGAPA-UNAM under grant PAPIIT IN100918. DMk acknowledges his work as part of the research activity of the National Astronomical Research Institute of Thailand (NARIT), which is supported by the Ministry of Science and Technology of Thailand. MR acknowledges the support of the French Agence Nationale de la Recherche (ANR), under grant ESRR (ANR-16-CE31-0007-01). We acknowledge the International Space Science Institute (ISSI) for supporting the SoFAR international team http://www.issi.unibe.ch/teams/sofar/.TW acknowledges the NSFC of China (Grant Nos. 11873084 and 11521303) and YunnanApplied Basic Research Projects (GrantNo. 2017B008). IS acknowledges for a partial support of DN 08-1/2016 funded by the Bulgarian NSF. This work hasmade use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, ht tps://www.cosmos.esa.int/web/gaia/dpac/consortium).Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 670519: MAMSIE) and from the Fonds Wetenschappelijk Onderzoek - Vlaanderen (FWO) under the grant agreement G0H5416N (ERC Opvangproject). This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI:10.26093/cds/vizier). The original description of the VizieR service was published in A&AS 143, 23. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

The appendix tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A117 ; Context. The main goal of the CARMENES survey is to find Earth-mass planets around nearby M-dwarf stars. Seven M dwarfs included in the CARMENES sample had been observed before with HIRES and HARPS and either were reported to have one short period planetary companion (GJ 15 A, GJ 176, GJ 436, GJ 536 and GJ 1148) or are multiple planetary systems (GJ 581 and GJ 876). Aims. We aim to report new precise optical radial velocity measurements for these planet hosts and test the overall capabilities of CARMENES. Methods. We combined our CARMENES precise Doppler measurements with those available from HIRES and HARPS and derived new orbital parameters for the systems. Bona-fide single planet systems were fitted with a Keplerian model. The multiple planet systems were analyzed using a self-consistent dynamical model and their best fit orbits were tested for long-term stability. Results. We confirm or provide supportive arguments for planets around all the investigated stars except for GJ 15 A, for which we find that the post-discovery HIRES data and our CARMENES data do not show a signal at 11.4 days. Although we cannot confirm the super-Earth planet GJ 15 Ab, we show evidence for a possible long-period (P = 7030 d) Saturn-mass (msini = 51.8M) planet around GJ 15 A. In addition, based on our CARMENES and HIRES data we discover a second planet around GJ 1148, for which we estimate a period P = 532.6 days, eccentricity e = 0.342 and minimum mass msini = 68.1M. Conclusions. The CARMENES optical radial velocities have similar precision and overall scatter when compared to the Doppler measurements conducted with HARPS and HIRES. We conclude that CARMENES is an instrument that is up to the challenge of discovering rocky planets around low-mass stars.© ESO, 2018. ; CARMENES is an instrument for the Centro Astronomico Hispano-Aleman de Calar Alto (CAHA, Almeria, Spain). CARMENES is funded by the German Max-Planck-Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Cientificas (CSIC), the European Union through FEDER/ERF FICTS-2011-02 funds, and the members of the CARMENES Consortium (Max-Planck-Institut fur Astronomie, Instituto de Astrofisica de Andalucia, Landessternwarte Konigstuhl, Institut de Ciencies de l'Espai, Insitut fur Astrophysik Gottingen, Universidad Complutense de Madrid, Thuringer Landessternwarte Tautenburg, Instituto de Astrofisica de Canarias, Hamburger Sternwarte, Centro de Astrobiologia and Centro Astronomico Hispano-Aleman), with additional contributions by the Spanish Ministry of Economy, the German Science Foundation (DFG), the Klaus Tschira Stiftung, the states of Baden-Wurttemberg and Niedersachsen, the DFG Research Unit FOR2544 >Blue Planets around Red Stars>, and by the Junta de Andalucia. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work used the Systemic Console package (Meschiari et al. 2009) for cross-checking our Keplerian and Dynamical fits and the python package astroML (VanderPlas et al. 2012) for the calculation of the GLS periodogram. The IEEC-CSIC team acknowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) and the Fondo Europeo de Desarrollo Regional (FEDER) through grant ESP2016-80435-C2-1-R, as well as the support of the Generalitat de Catalunya/CERCA programme. The IAA-CSIC team acknowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) through grants AYA2014-54348-C03-01 and AYA2016-79425-C3-3-P as well as FEDER funds. The UCM team acknowledges support by the Spanish Ministry of Economy and Competitiveness (MINECO) from projects AYA2015-68012-C2-2-P and AYA2016-79425- C3-1,2,3-P and the Spanish Ministerio de Educacion, Cultura y Deporte, programa de Formacion de Profesorado Universitario, under grant FPU15/01476. T. T. and M.K. thank to Jan Rybizki for the very helpful discussion in the early phases of this work. V.J.S.B. is supported by grant AYA2015-69350-C3-2-P from the Spanish Ministry of Economy and Competiveness (MINECO). J.C.S. acknowledges funding support from Spanish public funds for research under project ESP2015-65712-C5-5-R (MINECO/FEDER), and under Research Fellowship program >Ramon y Cajal> with reference RYC2012-09913 (MINECO/FEDER). The contributions of M.A. were supported by DLR (Deutsches Zentrum fur Luft- und Raumfahrt) through the grants 50OW0204 and 50OO1501. J.L.-S. acknowledges the Office of Naval Research Global (award No. N62909-15- 1-2011) for support. C.d.B. acknowledges that this work has been supported by Mexican CONACyT research grant CB-2012-183007 and the Spanish Ministry of Economy and Competitivity through projects AYA2014-54348-C3-2-R. J.I.G.H., and R.R. acknowledge financial support from the Spanish Ministry project MINECO AYA2014-56359-P. J.I.G.H. also acknowledges financial support from the Spanish MINECO under the 2013 Ramon y Cajal program MINECO RYC-2013-14875. V. Wolthoff acknowledges funding from the DFG Research Unit FOR2544 >Blue Planets around Red Stars>, project No. RE 2694/4-1.We thank the anonymous referee for the excellent comments that helped to improve the quality of this paper.

The CARMENES radial velocity (RV) survey is observing 324 M dwarfs to search for any orbiting planets. In this paper, we present the survey sample by publishing one CARMENES spectrum for each M dwarf. These spectra cover the wavelength range 520¿1710 nm at a resolution of at least R >80 000, and we measure its RV, H¿ emission, and projected rotation velocity. We present an atlas of high-resolution M-dwarf spectra and compare the spectra to atmospheric models. To quantify the RV precision that can be achieved in low-mass stars over the CARMENES wavelength range, we analyze our empirical information on the RV precision from more than 6500 observations. We compare our high-resolution M-dwarf spectra to atmospheric models where we determine the spectroscopic RV information content, Q, and signal-to-noise ratio. We find that for all M-type dwarfs, the highest RV precision can be reached in the wavelength range 700¿900 nm. Observations at longer wavelengths are equally precise only at the very latest spectral types (M8 and M9). We demonstrate that in this spectroscopic range, the large amount of absorption features compensates for the intrinsic faintness of an M7 star. To reach an RV precision of 1 m s¿1 in very low mass M dwarfs at longer wavelengths likely requires the use of a 10 m class telescope. For spectral types M6 and earlier, the combination of a red visual and a near-infrared spectrograph is ideal to search for low-mass planets and to distinguish between planets and stellar variability. At a 4 m class telescope, an instrument like CARMENES has the potential to push the RV precision well below the typical jitter level of 3-4 m s-1. © ESO 2018. ; We thank an anonymous referee for prompt attention and helpful comments that helped to improve the quality of this paper. CARMENES is an instrument for the Centro Astronomico Hispano-Aleman de Calar Alto (CAHA, Almeria, Spain). CARMENES is funded by the German Max-Planck-Gesellschaft (MPG), the Spanish Consejo Superior de Investigaciones Cientificas (CSIC), the European Union through FEDER/ERF FICTS-2011-02 funds, and the members of the CARMENES Consortium (Max-Planck-Institut fur Astronomie, Instituto de Astrofisica de Andalucia, Landessternwarte Konigstuhl, Institut de Ciencies de l'Espai, Insitut fur Astrophysik Gottingen, Universidad Complutense de Madrid, Thuringer Landessternwarte Tautenburg, Instituto de Astrofisica de Canarias, Hamburger Sternwarte, Centro de Astrobiologia and Centro Astronomico Hispano-Aleman), with additional contributions by the Spanish Ministry of Economy, the German Science Foundation through the Major Research Instrumentation Programme and DFG Research Unit FOR2544 >Blue Planets around Red Stars>, the Klaus Tschira Stiftung, the states of Baden-Wurttemberg and Niedersachsen, and by the Junta de Andalucia. This work has made use of the VALD database, operated at Uppsala University, the Institute of Astronomy RAS in Moscow, and the University of Vienna. We acknowledge the following funding programs: European Research Council (ERC-279347), Deutsche Forschungsgemeinschaft (RE 1664/12-1, RE 2694/4-1), Bundesministerium fur Bildung und Forschung (BMBF-05A14MG3, BMBF-05A17MG3), Spanish Ministry of Economy and Competitiveness (MINECO, grants AYA2015-68012-C2-2-P, AYA2016-79425-C3-1,2,3-P, AYA2015-69350-C3-2-P, AYA2014-54348-C03-01, AYA2014-56359-P, AYA2014-54348-C3-2R, AYA2016-79425-C3-3-P and 2013 Ramon y Cajal program RYC-2013-14875), Fondo Europeo de Desarrollo Regional (FEDER, grant ESP2016-80435-C2-1-R, ESP2015-65712-C5-5-R), Generalitat de Catalunya/CERCA programme, Spanish Ministerio de Educacion, Cultura y Deporte, programa de Formacion de Profesorado Universitario (grant FPU15/01476), Deutsches Zentrum fur Luft- und Raumfahrt (grants 50OW0204 and 50OO1501), Office of Naval Research Global (award no. N62909-15-1-2011), Mexican CONACyT grant CB-2012-183007.