2862 20160701171712.0 ASTROimport-1019 Blomme, R. 2010 The Colliding Winds of Cyg OB2 No. 8A Cyg OB2 No. 8A is an O6 If + O5.5 III(f) colliding-wind binary with a period of 21.9 days. Our VLA radio observations have detected non-thermal radiation that is due to the synchrotron emission from the colliding-wind region. The radio fluxes vary in a correlated way with orbital phase, which is quite unexpected, as all synchrotron emission should to be absorbed by the free-free absorption in the stellar wind material. We develop radiative transfer models for this system, which show that this orbit-locked variability can indeed be explained. This type of radio-detectable binary is important for binary statistics and for studying clumping and porosity in the stellar wind. Hot and Cool: Bridging Gaps in Massive Star Evolution 425 2010 289 http://esoads.eso.org/abs/2010ASPC..425..289B published in NONREF