ξ1 CMa: An Extremely Slowly Rotating Magnetic B0.7 IV Star

We present our analysis of 6 years of ESPaDOnS spectropolarimetry of the magnetic β Cep star ξ¹ 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.