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Description
The Cyg OB2 #5 system is thought to consist of a short-period (6.6d) eclipsing massive binary associated with an OB-star orbiting it with a period of ~6.7yr, themselves orbited by a distant early B-star (with thousands of years period). However, while the inner binary has been studied many times, information is missing on the other stars, in particular the third one whose presence was indirectly postulated from recurrent modulations in the radio domain. Besides, to this date, the X-ray lightcurve could not be fully interpreted, e.g. in the framework of colliding-wind emission linked to one of the systems. We obtained new optical and X-ray observations of Cyg OB2 #5, which we combined to archival data. We performed a thorough and homogeneous investigation of all available data, notably revisiting the times of primary minimum in photometry. In the X-ray domain, XMM-Newton provides scattered exposures over ~5000d whilst Swift provides a nearly continuous monitoring for the last couple of years. Though the X-ray lightcurve reveals clear variability, no significant period can be found hence the high-energy emission cannot be explained solely in terms of colliding winds varying along either the short or intermediate orbits. The optical data reveal for the first time clear signs of reflex motion: the photometry indicates the presence of a 2366d (i.e. 6.5yr) period while the associated radial velocity changes are detected at the 3{sigma} level in the systemic velocity of the HeII 4686 emission line. With the revised period, the radio lightcurve is interpreted consistently in terms of a wind interaction between the inner binary and the tertiary star. From the optical and radio data, we derive constraints on the physical properties of the tertiary star and its orbit.
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