2665 20160701171708.0 ASTROimport-822 Blomme, R. 2005 Observations of non-thermal radio emission in O-type stars The non-thermal radio emission of O-type stars is due to synchrotron radiation from relativistic electrons. These electrons are accelerated in shocks. Most probably, these shocks are due to colliding winds in a binary or multiple system. We present new VLA observations of the non-thermal radio emitter Cyg OB2 No. 8A, which was only recently discovered to be a binary. For this star, the radio fluxes show a good correlation with the phase in the binary period. This result is actually quite surprising because the high free-free optical depth should prevent us from detecting the synchrotron radiation emitted by the colliding-wind region. We can only explain the observations by introducing porosity in the model. We also present a complete series of archive VLA observations on the triple system HD 167971. The radio lightcurve shows a more complicated behaviour than for Cyg OB2 No. 8A: it is difficult to detect the binary period in the observations, but we found a clear variation that occurs on a ˜ 20-year timescale. Massive Stars and High-Energy Emission in OB Associations 2005 45-48 http://esoads.eso.org/abs/2005mshe.work...45B published in NONREF