The magnetic field of $\zeta$ Ori A
  Magnetic fields play a significant role in the evolution of massive stars. About 7% of massive stars are found to be magnetic at a level detectable with current instrumentation and only a few magnetic O stars are known. Detecting magnetic field in O stars is particularly challenging because they only have few, often broad, lines to measure the field, which leads to a deficit in the knowledge of the basic magnetic properties of O stars. We present new spectropolarimetric Narval observations of $\zeta$ Ori A. We also provide a new analysis of both the new and older data taking binarity into account. The aim of this study was to confirm the presence of a magnetic field in $\zeta$ Ori A. We identify that it belongs to $\zeta$ Ori Aa and characterize it. 
A. Blaz\`ere, C. Neiner, J-C. Bouret, A. Tkachenko and the MiMeS   collaboration
astro-ph.SR