2009A&A...495..231L

The properties of the early-type binary CygOB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. We have attempted to constrain the physical properties of the binary by collecting new optical and X-ray observations.The optical light curves obtained with narrow-band continuum and line-bearing filters are analysed and compared. Optical spectra are used to map the location of the HeII λ4686 and Hα line-emission regions in velocity space. New XMM-Newton as well as archive X-ray spectra are analysed to search for variability and constrain the properties of the hot plasma in this system.We find that the orbital period of the system slowly changes though we are unable to discriminate between several possible explanations of this trend. The best fit solution of the continuum light curve reveals a contact configuration with the secondary star being significantly brighter and hotter on its leading side facing the primary. The mean temperature of the secondary star turns out to be only slightly lower than that of the primary, whilst the bolometric luminosity ratio is found to be 3.1. The solution of the light curve yields a distance of 925±25pc much lower than the usually assumed distance of the CygOB2 association. Whilst we confirm the existence of episodes of higher X-ray fluxes, the data reveal no phase-locked modulation with the 6.6day period of the eclipsing binary nor any clear relation between the X-ray flux and the 6.7yr radio cycle.The bright region of the secondary star is probably heated by energy transfer in a common envelope in this contact binary system as well as by the collision with the primary's wind. The existence of a common photosphere probably also explains the odd mass-luminosity relation of the stars in this system. Most of the X-ray, non-thermal radio, and possibly γ-ray emission of CygOB2 #5 is likely to arise from the interaction of the combined wind of the eclipsing binary with at least one additional star of this multiple system.