Non-thermal emission in Wolf-Rayet stars: are massive companions required?

We examine the radio spectral indices of 23 Wolf-Rayet (WR) stars to identify the nature of their radio emission. We identify nine systems as non-thermal emitters. In seven of these systems the non-thermal emission dominates the radio spectrum, while in the remaining two it is of comparable strength to the thermal, stellar wind emission, giving 'composite' spectra. Among these nine systems, seven have known spectroscopic or visual binary companions. The companions are all massive O or early B-type stars, strongly supporting a connection between the appearance of non-thermal emission in WR stars and the presence of a massive companion. In three of these binaries, the origin of non-thermal emission in a wind-collision region between the stars has been well established in earlier work. The binary systems that exhibit only thermal emission are all short-period systems where a wind-collision zone is deep within the opaque region of the stellar wind of the WR star. To detect non-thermal emission in these systems requires optically thin lines of sight to the wind-collision region.