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/* Dwarf Novae */ comma needed
Normally, the observed light comes from four distinct sources in the binary system: the white dwarf primary, the cooler secondary, the accretion disk, and a hot spot. This result occurs when a binary pair (a white dwarf primary and its secondary star, typically a subgiant) have a mass between 0.5 and 1 solar masses and are locked in a very close orbit (the orbit taking only between 3 and 15 hours). As they orbit, matter from the companion collects around the white dwarf in an accretion disk forming a hot spot. From the point of an observer this gives the appearance of a single source of light.
Although U Geminorum stars typically have routine small variations in light over time. it is the unpredictable, sudden increase in order of magnitude that also classify these stars as ''dwarf novae''. There are two competing theories at present to explain these outbursts. The first, called the ''mass-transfer burst model'' is where a sudden increase of material is transferred from the secondary companion to the white dwarf, causing the accretion disk to collapse, dumping a large amount of [[hydrogen]] on the surface of the white dwarf. It is thought that this tremendous release of energy through the sudden mass-transfer is the source of the outburst and corresponding sudden rise in magnitude. The other theory, called the ''disk-instability model'' , maintains that gas from the secondary subgiant builds up on the outer edges of the accretion disk around the white dwarf. When critical density occurs on this outer edge, thermal instabilities within the disk causes the matter to collapse onto the surface of the white dwarf itself. This release of energy from the accretion disk's instability releases gravitational energy, which is the source of the sudden corresponding rise in magnitude. <ref>
*http://www.daviddarling.info/encyclopedia/U/U_Geminorum_star.html
*http://home.mindspring.com/~mikesimonsen/cvnet/id1.html
