We report the detection of a strong , organized magnetic field in the helium-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 helium-strong star , with v \sin i = 300 \pm 15 km s ^ { -1 } and a rotational period of 0.5214404 \pm 0.0000006 days .
We have modeled our eight longitudinal magnetic field measurements assuming an oblique dipole magnetic field .
Constraining the inclination of the rotation axis to be between 38 ^ { \circ } and 86 ^ { \circ } , we find the magnetic obliquity angle to be between 30 ^ { \circ } and 85 ^ { \circ } , 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 behavior coupled with previously-reported variable emission of the H \alpha line ( which we confirm ) strongly supports the proposal that HR 7355 harbors a structured magnetosphere similar to that in the prototypical helium-strong star , \sigma Ori E .