any of a class of irregular variable stars that display sudden increases in brightness so great that they are sometimes called dwarf novae. Some have been observed to brighten by as much as 5 magnitudes (100 times) in a period of hours. The prototype star, U Geminorum, brightens by as much as 4 magnitudes (about 40 times) in a few days, declining again to normal brightness in two or three weeks. U Geminorum was the first of the class to be discovered, in 1855, by the English astronomer John Russell Hind. Fewer than 200 U Geminorum stars are known; sometimes they are called SS Cygni stars, after the brightest known member of the class. All are binaries with periods of less than a day.
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