Published online by Cambridge University Press: 23 January 2015
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 Prot > 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 Bd > 6 kG, a strength also inferred by the best-fit sinusoids to the longitudinal magnetic field measurements BZ when phased with a 60-year Prot.