Radio observations of candidate magnetic O stars

A number of O stars are suspected to have (weak) magnetic fields because of the observed cyclical variability in their UV wind-lines. However, direct detections of these magnetic fields with optical spectropolarimetry have proven to be very difficult. We have searched for non-thermal radio emission, which would be a strong indication for the presence of a magnetic field. Of our 5 selected candidate magnetic O stars, 3 are detected: $\xi$ Per, which we find to have a non-thermal spectrum, and $\lambda$ Cep and $\alpha$ Cam which show a thermal spectrum. We also find that the fluxes are lower than the expected free-free (thermal) contribution of the stellar wind. This is in agreement with recent findings that the mass-loss rates from O stars using H$\alpha$ are overestimated because of clumping in the inner part of the stellar wind.