A type of cataclysmic binary showing abrupt outbursts at unpredictable intervals, brightening within a day with typical amplitudes of 3–8 mag.; abbr. UG. They are also known as dwarf novae. A U Geminorum star is an interacting binary, with mass transfer from the main-sequence or slightly evolved secondary to an accretion disk around the white-dwarf primary. Eclipses are observed in some systems, together with a hump caused by a hot spot (1), located either where the gas stream hits the disk or at the inner edge of the disk. Orbital periods range from 80 minutes to 14 hours, and both components have masses similar to that of the Sun. Specific subtypes are SS Cygni stars, SU Ursae Majoris stars, and Z Camelopardalis stars.