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. 
R. S. Schnerr, K. L. J. Rygl, A. J. van der Horst, T. A. Oosterloo, J.   C. A. Miller-Jones, H. F. Henrichs