2010MNRAS.405L..51O

We report the detection of a strong, organized magnetic field in the He-variable early B-type star HR 7355 using spectropolarimetric data obtained with ESPaDOnS on the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is both the most rapidly rotating known main-sequence magnetic star and the most rapidly rotating He-strong star, with v sin i = 300±15 km/s and a rotational period of 0.5214404±0.0000006 d. We have modelled our eight longitudinal magnetic field measurements assuming an oblique dipole magnetic field. Constraining the inclination of the rotation axis to be between 38° and 86°, we find the magnetic obliquity angle to be between 30° and 85°, and the polar strength of the magnetic field at the stellar surface to be between 13-17 kG. The photometric light curve constructed from HIPPARCOS archival data and new CTIO measurements shows two minima separated by 0.5 in rotational phase and occurring 0.25 cycles before/after the magnetic extrema. This photometric behaviour, coupled with previously reported variable emission of the Hα line (which we confirm), strongly supports the proposal that HR 7355 harbours a structured magnetosphere similar to that in the prototypical He-strong star, σ Ori E.