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Context: Nonthermal radio emission in massive stars is expected to arise inwind-wind collisions occurring inside a binary system. One such case, theO-type star Cyg OB2 #9, was proven to be a binary only four years ago, but theorbital parameters remained uncertain. The periastron passage of 2011 was thefirst one to be observable under good conditions since the discovery ofbinarity. Aims: In this context, we have organized a large monitoring campaignto refine the orbital solution and to study the wind-wind collision. Methods:This paper presents the analysis of optical spectroscopic data, as well as of adedicated X-ray monitoring performed with Swift and XMM. Results: In light ofour refined orbital solution, Cyg OB2 #9 appears as a massive O+O binary with along period and high eccentricity; its components (O5-5.5I for the primary andO3-4III for the secondary) have similar masses and similar luminosities. Thenew data also provide the first evidence that a wind-wind collision is presentin the system. In the optical domain, the broad Ha line varies, displayingenhanced absorption and emission components at periastron. X-ray observationsyield the unambiguous signature of an adiabatic collision because, as the starsapproach periastron, the X-ray luminosity closely follows the 1/D variationexpected in that case. The X-ray spectrum appears, however, slightly softer atperiastron, which is probably related to winds colliding at slightly lowerspeeds at that time. Conclusions: It is the first time that such a variationhas been detected in O+O systems, and the first case where the wind-windcollision is found to remain adiabatic even at periastron passage.

The 2.35 year itch of Cyg OB2 #9 - I. Optical and X-ray monitoring