Hipparcos, VLA, and CCD observations of Cygnus OB2 No. 5: Solving the mystery of the radio ''companion''

We present accurate optical and radio astrometry for the unresolved contact binary system Cyg OB2 No. 5 and its weaker component to the northeast. While the radio and optical positions of the primary component (taken to be the unresolved contact binary) agree within observational error (similar to 70 mas), we find that the weaker radio component does not agree in position with the second optical component of the system, but falls in between the two optical components, We conclude that the weaker radio component of this system is not associated with the secondary star but appears to be synchrotron emission produced at the shock interaction zone between the winds of the stars. Differential B-and V-band CCD and Hipparcos photometry was used to derive the approximate spectral type (B0 V-B2 V) of the secondary. Ram pressure arguments in the colliding-wind model give a mass-loss rate for the secondary star consistent with the expected one, considering the earliest spectral type.