A type of U Geminorum star that occasionally exhibits a supermaximum (also called a superoutburst), i.e. an outburst that is about 2 magnitudes brighter and five times as long as a normal maximum; abbr. UGSU. The intervals and properties of normal maxima are similar to those found in an SS Cygni star, and are three to four times as frequent as supermaxima. Supermaxima show a pronounced periodicity, unlike all other outbursts of dwarf novae. During a supermaximum, a brightening known as a superhump is visible in the light-curve. This feature has an amplitude of about 0.2–0.3 mag., and a period that is a few per cent longer than the binary’s orbital period (which is less than 0.1 day). The interval between supermaxima is typically a few hundred days. However, some systems, the ER Ursae Majoris stars, have much shorter intervals (20–50 days) while others, the WZ Sagittae stars, have very long intervals, the most extreme being WZ Sagittae itself, with an interval of around 12 000 days.