Rapid Oscillations in Cataclysmic Variables. IX. BG Canis Minoris (=3A 0729+103)

We report the results of a 10-yr photometric program on BG Canis Minoris, a nova-like variable distinguished by the presence of a strictly coherent 913-s wave in its light curve. We present long-term ephemerides for the orbital clock, which manifests itself through a 3.2-h light modulation, and the 913-s clock. The former is found to be quite stable, and may originate from partial obscuration of the white dwarf by a bloated bright spot at the outer edge of the disk. The rapid oscillation shows a period decreasing on a time scale of 4 × 10⁵ yr, consistent with the idea that a magnetic white dwarf is spinning up under the torques exerted by accreting gas. The Ṗ measurement requires Ṃ>7 × 10⁻¹⁰ M⊙ yr⁻¹; combining the constraints from Ñ and the polarization, we estimate a magnetospheric radius about equal to the corotation radius, with Ṃ ≈ 10⁻⁹M⊙ yr⁻¹. Power spectra of the light curves reveal evidence for other high-frequency signals which seem to be related to the 913-s pulse. Features that may be stable in period, though not in amplitude, are found at 1073 ± 8 and 1977 ± 10 s. The former probably arises from reprocessing of the pulsed light in structures which are in prograde orbit around the white dwarf. We find no particularly simple way to understand the other peak, but there is a substantial hint that the true rotation period of the white dwarf is 1827 s, not 913 s.