  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 $P_{\rm rot}$.
