New V-band photometry of Cyg OB2 5 (V729 Cyg)

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.