The non-thermal, time-variable radio emission from Cyg OB2 # 5: A
  wind-collision region
  The radio emission from the well-studied massive stellar system Cyg OB2 #5 is
known to fluctuate with a period of 6.7 years between a low-flux state when the
emission is entirely of free-free origin, and a high-flux state when an
additional non-thermal component (of hitherto unknown nature) appears. In this
paper, we demonstrate that the radio flux of that non-thermal component is
steady on timescales of hours, and that its morphology is arc-like. This shows
that the non-thermal emission results from the collision between the strong
wind driven by the known contact binary in the system, and that of an unseen
companion on a somewhat eccentric orbit with a 6.7-yr period and a 5 to 10 mas
semi-major axis. Together with the previously reported wind-collision region
located about 0.8 arcsec to the north-east of the contact binary, Cyg OB2 #5
appears to be the only multiple system known so far to harbor two radio-imaged
wind-collision regions.
