xi¹ CMa: An Extremely Slowly Rotating Magnetic B0.7 IV Star

We present our analysis of 6 years of ESPaDOnS spectropolarimetry of the magnetic $\beta$ Cep star $\xi^1$ CMa (B0.7 IV). This high-precision magnetometry is consistent with a rotational period $P{\rm rot} >$ 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 $B_{\rm d} > 6$ kG, a strength also inferred by the best-fit sinusoids to the longitudinal magnetic field measurements $B_{\rm Z}$ when phased with a 60-year $P_{\rm rot}$.