U Geminorum Stars

Type of variable star displaying sudden and unpredictable increases in brightness, sometimes referred to as dwarf novae. Brightness may increase by as much as 2–5 magnitudes in hours, followed by a slower fading over days to weeks. U Geminorum stars are a binary system consisting of a white dwarf and a red giant that has filled its Roche-lobe. Roche-lobe overflow causes matter to fall onto an accretion disk around the white dwarf creating a hot spot at the point of impact. Thus, the light from a U Geminorum star has at least four sources: the two stars, the accretion disk and hot spots on the accretion disk. Sudden increases in brightness probably reflect increases in the amount of material falling onto the white dwarf from the accretion disk.

Image source: http://astronomy.swin.edu.au/cms/astro/cosmos/U/U+Geminorum+Stars.

The causes of sudden increases in accretion rate are not fully understood. One possibility is that sudden increase in the transfer of material from the companion star to the accretion disk causes the disk to collapse onto the white dwarf. Another, preferred, possibility is that the accretion disk can only accumulate a finite amount of gas before becoming unstable and instability causes an abrupt increase in the amount of matter falling onto the white dwarf.