  In colliding-wind binaries, shocks accelerate a fraction of the electrons up
to relativistic speeds. These electrons then emit synchrotron radiation at
radio wavelengths. Whether or not we detect this radiation depends on the size
of the free-free absorption region in the stellar winds of both components. One
expects long-period binaries to be detectable, but not the short-period ones.
It was therefore surprising to find that Cyg OB2 No. 8A (P = 21.9 d) does show
variability locked with orbital phase. To investigate this, we developed a
model for the relativistic electron generation (including cooling and
advection) and the radiative transfer of the synchrotron emission through the
stellar wind. Using this model, we show that the synchrotron emitting region in
Cyg OB2 No. 8A does extend far enough beyond the free-free absorption region to
generate orbit-locked variability in the radio flux. This model can also be
applied to other non-thermal emitters and will prove useful in interpreting
observations from future surveys, such as COBRaS - the Cyg OB2 Radio Survey.
