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 \pm 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 \leq 0 \hbox to 0.0 pt { . } { { } ^ { \prime \prime } } 02 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 .