We present our analysis of 6 years of ESPaDOnS spectropolarimetry of the magnetic \beta Cep star \xi ^ { 1 } CMa ( B1 III ) .
This high-precision magnetometry is consistent with a rotational period P _ { rot } > 40 yr. Absorption line profiles can be reproduced with a non-rotating model .
We constrain R _ { * } , L _ { * } , and the stellar age via a Baade-Wesselink analysis .
Spindown due to angular momentum loss via the magnetosphere predicts an extremely long rotational period if the magnetic dipole B _ { d } > 6 kG , a strength also inferred by the best-fit sinusoids to the longitudinal magnetic field measurements B _ { Z } when phased with a 60-year P _ { rot } .