On the nature of cyclic light variations in UX Ori stars

Based on long-term series of photoelectric observations of the UX Ori stars BF Ori, RR Tau, and CO Ori, we obtained evidence for the cyclicity of thei r photometric activity on a time scale from several years to ten years or m ore. Since the photometric variability of this type of star is attributable to variable circumstellar extinction and is determined by the instantaneou s dust column density, the presence of activity cycles points to the existe nce in circumstellar dust disks of large-scale nonuniformities in the dust distribution which periodically appear on the Line of sight. The components of a binary (or multiple) system or protoplanets can be the source of such nonuniformities.