Regularities in the long-term optical light curve of the black hole candidate binary A0620 - 00 (V616 Mon)

We have monitored the R and I magnitudes of the black hole candidate system A0620 - 00 (V616 Mon) in the years 1991-1995 at the Wise Observatory. Combining our data with some additional measurements, we analyse a sparsely covered 7-yr light curve of the star. We find that the average R-band magnitude varies on a time-scale of a few hundred days, with a peak-to-peak amplitude of 0.3 mag. The two maxima in the well-known double hump binary cycle, as well as one of the minima between them, vary by a few per cent relative to one another, in a seemingly random way. One maximum is, on average, higher by 0.05 mag than the other. The depth of the second minimum varies with significantly higher amplitude, in clear correlation with the long-term variability of the mean magnitude of the system. It is shallower than the other minimum at times of maximum light. It deepens when the system brightness declines, and it becomes the deeper of the two minima at times of minimum system light. According to the commonly acceptable phasing of the binary cycle, the systematically varying minimum corresponds to the inferior conjunction of the red dwarf. We cannot suggest any simple geometrical model for explaining the regularities that we find in the long-term photometric behaviour of the V616 Mon binary system.