Recent BRITE-Constellation space photometry of the slowly rotating , magnetic \beta Cep pulsator \xi ^ { 1 } CMa permits a new analysis of its pulsation properties .
Analysis of the two-colour BRITE data reveals the well-known single pulsation period of 0.209 d , along with its first and second harmonics .
A similar analysis of SMEI and TESS observations yields compatible results , with the higher precision TESS observations also revealing several low-amplitude modes with frequencies below 5 d ^ { -1 } ; some of these are likely g modes .
The phase lag between photometric and radial velocity maxima - equal to 0.334 cycles - is significantly larger than the typical value of 1 / 4 observed in other large-amplitude \beta Cep stars .
The phase lag , as well as the strong dependence of phase of maximum light on wavelength , can be reconciled with seismic models only if the dominant mode is the fundamental radial mode .
We employ all published photometric and radial velocity measurements , spanning over a century , to evaluate the stability of the pulsation period .
The O - C diagram exhibits a clear parabolic shape consistent with a mean rate of period change \dot { P } = 0.34 \pm 0.02 s/cen .
The residuals from the best-fit parabola exhibit scatter that is substantially larger than the uncertainties .
In particular , dense sampling obtained during the past \sim 20 years suggests more complex and rapid period variations .
Those data can not be coherently phased with the mean rate of period change , and instead require \dot { P } \sim 0.9 s/cen .
We examine the potential contributions of binarity , stellar evolution , and stellar rotation and magnetism to understand the apparent period evolution .