Are all non-thermal radio-emitting O-stars binaries?

We present qualitative models for the non-thermal radio emission of single 0 stars, in terms of synchrotron emission from relativistic electrons. These electrons are accelerated in shocks generated by the instability of the driving mechanism of the stellar wind. Hydrodynamical simulations show that wind-embedded shocks decay as they move outward with the flow. When we include this effect in our synchrotron models, we find that the observed non-thermal emission cannot be explained by synchrotron emission from wind-embedded shocks. The most likely alternative is that relativistic electrons are accelerated in shocks generated by the collision of two stellar winds. The nonthermal radio emitters are therefore all binary or multiple systems. This hypothesis is supported by periodic radio light curves of two presumably single O stars