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Published online by Cambridge University Press: 12 April 2016
SU UMa stars are one of subclasses of dwarf novae. Dwarf novae are semi-detached close binary systems in which a Roche-lobe filling red dwarf secondary loses matter and the white dwarf primary accretes it through the accretion disk. The main characteristics of SU UMa subclass is that they show two kinds of outbursts: normal outbursts and superoutbursts. In addition to the more frequent narrow outbursts of normal dwarf nova, SU UMa stars exhibit “superoutbursts”, in which stars reach about 1 magnitude brighter and stay longer than in normal outburst. Careful photometric studies during superoutburst have almost always revealed the “superhumps”: periodic humps in light curves with a period very close to the orbital period of the system. However, the most curious of all is that this superhump period is not exactly equal to the orbital period, but it is always longer by a few percent than the orbital period.