Long-time photometric study of UX Orionis stars

Results from long-term optical photometric observations of the pre-main sequence stars from UX Orionis type are reported. The UBVRI CCD observations were performed in two observatories with four telescopes: the 2-m Ritchey-Chrètien-Coudè, the 50/70-cm Schmidt and the 60-cm Cassegrain telescopes of the National Astronomical Observatory Rozhen (Bulgaria) and the 1.3-m Ritchey-Crètien telescope of the Skinakas Observatory in Crete (Greece). The pre-main sequence stars from UX Ori type show strong photometric variability with sudden quasi-Algol drops in brightness and amplitudes up to 3 mag. The widely accepted explanation of its variability is a variable extinction from dust clumps or filaments passing through the line of sight to the star. We present the results of observations on three objects of this type: V1184 Tau, GM Cep and V1180 Cas. The three pre-main sequence stars show deep drops in brightness of varying duration and amplitudes. While in the case of V1184 Tau a deep minimum in brightness lasting twelve years has been observed, the other two stars show several minima with different amplitudes and duration. Photometric data presented in this paper show the usefulness of systematically monitoring of pre-main sequence stars with large amplitude variability. By accumulation of enough photometric data, we can more accurately determine the causes of photometric variability of young stellar objects.