  The Cyg OB2 #5 system is thought to consist of a short-period (6.6 d)
eclipsing massive binary orbited by an OB-star orbiting with a period of ~6.7
yr; these stars in turn are orbited by a distant early B-star with a period of
thousands of years. However, while the inner binary has been studied many
times, information is missing on the other stars, in particular the third star
whose presence was indirectly postulated from recurrent modulations in the
radio domain. Besides, to this date, the X-ray light curve could not be fully
interpreted, for example 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 ~5000 d whilst Swift provides a nearly continuous monitoring for
the last couple of years. Although the X-ray light curve 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 2366 d (i.e.
6.5 yr) period while the associated radial velocity changes are detected at the
3 sigma level in the systemic velocity of the He II 4686 emission line. With
the revised period, the radio light curve is interpreted consistently in terms
of a wind interaction between the inner binary and the tertiary star. From
these optical and radio data, we derive constraints on the physical properties
of the tertiary star and its orbit.
