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 s ystem A0620 - 00 (V616 Mon) in the years 1991-1995 at the Wise Observa tory. Combining our data with some additional measurements, we analyse a sparsely covered 7-yr light curve of the star. We find that the ave rage R-band magnitude varies on a time-scale of a few hundred days, wi th a peak-to-peak amplitude of 0.3 mag. The two maxima in the well-kno wn 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 rando m way. One maximum is, on average, higher by 0.05 mag than the other. The depth of the second minimum varies with significantly higher ampli tude, in clear correlation with the long-term variability of the mean magnitude of the system. It is shallower than the other minimum at tim es of maximum light. It deepens when the system brightness declines, a nd it becomes the deeper of the two minima at times Of minimum system light. According to the commonly acceptable phasing of the binary cycl e, the systematically varying minimum corresponds to the inferior conj unction of the red dwarf. We cannot suggest any simple geometrical mod el for explaining the regularities that we find in the long-term photo metric behaviour of the V616 Mon binary system.