Orbital and spin variability of the intermediate polar BG CMi

Results of a CCD study of the variability of the cataclysmic variable BG CMi obtained at the Korean 1.8 m telescope in 2002-2005 are presented. The multi-comparison star method had been applied for better accuracy estimates. The linear ephemeris based on 19 mean maxima for 2002-2005 is HJD 2453 105.31448(6)+ 0.01057257716(198)(E-764707). The period differs from that predicted by the quadratic ephemeris by Pych et al. (1996, AcA, 46, 279) leading to a possible cycle miscount. The statistically optimal ephemeris is a fourth-order polynomial, as a quadratic or even a cubic ephemeris leads to unaceptably large residuals: Min.HJD = 2445 (120.28095(28) +0.0105729609(57)E - 1.58(32) x 10 -13 E 2 - 5.81(64) x 10 -19 E 3 + 4.92(41) x 10 -25 E 4 . Thus the rate of the spin-up of the white dwarf is decreasing. An alternative explanation is that the spin-up has been stopped during recent years. The deviations between the amplitudes of the spin variability in V and R, as well as between phases are not statistically significant. However, the orbital light curves exhibit distinct difference; the corresponding color index shows a nearly sinusoidal shape with a maximum at orbital phase ∼0.2. The variations of the amplitude of spin waves shows a short maximum at the phase of the orbital dip. The corrected ephemeris for orbital minima is Min.HJD = 2448 368.7225(12) + 0.13474841(6) . (E - 24849) with a narrow dip occuring 0.07P later. The rate of the spin period variation seems to be changed, justifying the necessity of regular observations of intermediate polars.