On the Size of the Non-Thermal Component in the Radio Emission from Cyg
  OB2 #5
  Cyg OB2 #5 is a contact binary system with variable radio continuum emission.
This emission has a low-flux state where it is dominated by thermal emission
from the ionized stellar wind and a high-flux state where an additional
non-thermal component appears. The variations are now known to have a period of
6.7 +/- 0.2 yr. The non-thermal component has been attributed to different
agents: an expanding envelope ejected periodically from the binary, emission
from a wind-collision region, or a star with non-thermal emission in an
eccentric orbit around the binary. The determination of the angular size of the
non-thermal component is crucial to discriminate between these alternatives. We
present the analysis of VLA archive observations made at 8.46 GHz in 1994 (low
state) and 1996 (high state), that allow us to subtract the effect of the
persistent thermal emission and to estimate an angular size of 0.02 arcseconds
for the non-thermal component. This compact size favors the explanation in
terms of a star with non-thermal emission or of a wind-collision region.
