A multi-frequency study of Wolf-Rayet stars: A case for long-period binaries

We discuss the incidence of Wolf-Rayet stars with variable non-thermal radio emission, excess X-ray fluxes, and episodic/periodic IR excesses. Two stars, WR140 and WR125, show all three phenomena, eleven show at least one of those phenomena. In WR140 (WC7+O4-5, P = 7.94 yr) we observe with VLBI its non-thermal 6cm radio emission from a localized area of 11% of the size of the 6cm free-free radio photosphere. We associate the non-thermal radio emission with the colliding winds of WR and O star in WR140. Generalizing the case of WR140, we argue that all thirteen objects are long-period WR+OB or WR+c binaries with orbit sizes of the order of magnitude of the sizes of the radio photospheres of the WR components or larger, where the non-thermal radio emission is associated with their colliding winds.