Mixing in magnetic OB stars

Recent observations have revealed the existence of a population of slowly-rotating, nitrogen-rich B dwarfs that are not predicted by evolutionary models including rotational mixing. However, as theoretical arguments suggest that magnetic processes may significantly increase the efficiency of the transport of the chemical elements, it is of importance to assess the extent of mixing in some known magnetic OB stars. We review our knowledge of the CNO abundance properties of these objects and present the first results of an NLTE abundance study of massive stars identified as being magnetic by the MiMeS collaboration. Although a nitrogen excess is often associated with the presence of a magnetic field, there is no evidence for a strict one-to- one correspondence between these two phenomena. This therefore suggests that other (still elusive) parameters may control the amount of mixing experienced by main-sequence OB stars.