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Context. It appears that most (if not all) massive stars are born in multiple systems. At the same time, the most massive binaries are hard to find owing to their low numbers throughout the Galaxy and the implied large distances and extinctions. Aims. We want to study LS III +46 11, identified in this paper as a very massive binary; another nearby massive system, LS III +46 12; and the surrounding stellar cluster, Berkeley 90. Methods. Most of the data used in this paper are multi-epoch high S/N optical spectra, although we also use Lucky Imaging and archival photometry. The spectra are reduced with dedicated pipelines and processed with our own software, such as a spectroscopic-orbit code, CHORIZOS, and MGB. Results. LS III +46 11 is identified as a new very early O-type spectroscopic binary [O3.5 If∗ + O3.5 If∗] and LS III +46 12 as another early O-type system [O4.5 V((f))]. We measure a 97.2-day period for LS III +46 11 and derive minimum masses of 38.80 ± 0.83 M and 35.60 ± 0.77 M for its two stars. We measure the extinction to both stars, estimate the distance, search for optical companions, and study the surrounding cluster. In doing so, a variable extinction is found as well as discrepant results for the distance. We discuss possible explanations and suggest that LS III +46 12 may be a hidden binary system where the companion is currently undetected. ; J.M.A. and A.S. acknowledge support from [a] the Spanish Government Ministerio de Economia y Competitividad (MINECO) through grants AYA2010-15 081, AYA2010-17631, and AYA2013-40611-P and [b] the Consejeria de Educacion of the Junta de Andalucia through grant P08-TIC-4075. J.M.A. was also supported by the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy and he is grateful to the Department of Physics and Astronomy at Texas A&M University for their hospitality during some of the time this work was carried out. I.N., A.M., J.A., and J.L. acknowledge support from [a] the Spanish Government Ministerio de Econoia y Competitividad (MINECO) through grant AYA2012-39364-C02-01/02, [b] the European Union, and [c] the Generalitat Valenciana through grant ACOMP/2014/129. R.H.B. acknowledges support from FONDECYT Project 1140076. S.S.-D. acknowledges funding by [a] the Spanish Government Ministerio de Economia y Competitividad (MINECO) through grants AYA2010-21697-C05-04, AYA2012-39 364-C02-01, and Severo Ochoa SEV-2011-0187 and [b] the Canary Islands Government under grant PID2 010 119. J.S.-B. acknowledges support by the JAE-PreDoc program of the Spanish Consejo Superior de Investigaciones Cientificas (CSIC). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. ; Peer Reviewed

We report magnetic and spectroscopic observations and modelling of the Of?p star CPD −28° 2561. Using more than 75 new spectra, we have measured the equivalent width variations and examined the dynamic spectra of photospheric and wind-sensitive spectral lines. A period search results in an unambiguous 73.41 d variability period. High-resolution spectropolarimetric data analysed using least-squares deconvolution yield a Zeeman signature detected in the mean Stokes V profile corresponding to phase 0.5 of the spectral ephemeris. Interpreting the 73.41 d period as the stellar rotational period, we have phased the equivalent widths and inferred longitudinal field measurements. The phased magnetic data exhibit a weak sinusoidal variation, with maximum of about 565 G at phase 0.5, and a minimum of about −335 G at phase 0.0, with extrema approximately in phase with the (double-wave) Hα equivalent width variation. Modelling of the Hα equivalent width variation assuming a quasi-3D magnetospheric model produces a unique solution for the ambiguous couplet of inclination and magnetic obliquity angles: (i, β) or (β, i) = (35°, 90°). Adopting either geometry, the longitudinal field variation yields a dipole polar intensity Bd = 2.6 ± 0.9 kG, consistent with that obtained from direct modelling of the Stokes V profiles. We derive a wind magnetic confinement parameter η* ≃ 100, leading to an Alfvén radius RA ≃ 3–5R*, and a Kepler radius RK ≃ 20R*. This supports a physical scenario in which the Hα emission and other line variability have their origin in an oblique, corotating 'dynamical magnetosphere' structure resulting from a magnetically channelled wind. Nevertheless, the details of the formation of spectral lines and their variability within this framework remain generally poorly understood. ; Fil: Wade, G. A. Royal Military College of Canada. Department of Physics; Canadá ; Fil: Barba, R. H. . Universidad de la Serena; Chile ; Fil: Grunhut, J. European Southern Observatories; Alemania ; Fil: Martins, F. Universite Montpellier Ii; Francia ; Fil: Petit, V. University of Delaware. Department of Physics and Astronomy. Bartol Research Institute; Estados Unidos ; Fil: Sundqvist, J. O. . Institut fur Astronomie und Astrophysik der Universität München; Alemania ; Fil: Townsend, R. H. D. . University of Wisconsin-Madison. Department of Astronomy; Estados Unidos ; Fil: Walborn, N. R. . Space Telescope Science Institute; Estados Unidos ; Fil: Alecian, E. UJF-Grenoble 1/CNRS-INSU. Institut de Planetologie et d'Astrophysique ´ de Grenoble (IPAG); Francia ; Fil: Alfaro, E. J. Consejo Superior de Investigaciones Cientificas. Instituto de Astrofísica de Andalucía; España ; Fil: Maíz Apellaniz, J. Consejo Superior de Investigaciones Cientificas. Instituto de Astrofísica de Andalucía; España ; Fil: Arias, Julia Ines. Universidad de la Serena; Chile ; Fil: Gamen, Roberto Claudio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; Argentina ; Fil: Morrell, Nidia Irene. Observatories of the Carnegie Institution of Washington. Las Campanas Observatory; Estados Unidos ; Fil: Naze, Y. Universitée de Liéege; Bélgica ; Fil: Sota, A. Consejo Superior de Investigaciones Cientificas. Instituto de Astrofísica de Andalucía; España ; Fil: ud-Doula, A. Penn State Worthington Scranton; Estados Unidos ; Fil: MiMeS Collaboration.