SPIN-RESOLVED OPTICAL CCD SPECTROSCOPY AND PHOTOMETRY OF BG-CANIS-MINORIS

We report spin-resolved optical photometry and spectroscopy of the intermediate polar BG Canis Minoris. New 913-s pulsation ephemerides are derived. We find that a cubic fit provides a better description of the O-C values that the hitherto assumed quadratic ephemeris at a confidence level of 97.3 per cent. From the spectroscopy, we find that the violet-to-red (V/R) ratio of the emission lines varies with the 913-s period but not at the alternative period of 847 s detected in the X-ray band. This suggests that 913 s is the most probable spin period of the white dwarf. However, we do find evidence that the emission line intensities and equivalent widths are modulated at 847 s. The morphology and phasing of the 913-s V/R ratio modulation with respect to the optical and X-ray light curves are similar to those of the spin modulation seen in other intermediate polars, and are consistent with the behaviour expected from material in an azimuthally extended magnetically constrained flow, formed at the inner edge of a truncated accretion disc. However, the confirmation of the 847-s period makes it difficult to accept this interpretation.