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cataclysmic variable(kat-ă-kliz -mik) (eruptive variable) A close binary star system where one member is a white dwarf, and mass transfer on to the latter causes sudden large and unpredictable changes in brightness. The main classes of cataclysmic variable are classical novae, recurrent novae, dwarf novae, and symbiotic stars. The outbursts of cataclysmic variables are also detected at ultraviolet and X-ray wavelengths; their X-ray output is however much less than that of X-ray transients, which are similar systems where the compact star is a neutron star rather than a white dwarf.
It is generally believed that the progenitors of cataclysmic variables are wide binaries with periods of several months to several years. When the primary evolves and fills its Roche lobe as a giant, mass is lost from it (see equipotential surfaces); this mass forms a common envelope surrounding the core of the giant (a white dwarf) and the companion. Due to frictional drag, the orbit of the immersed binary shrinks until the envelope is ejected forming a bright planetary nebula and a short-period precataclysmic binary.
The distribution of orbital periods of cataclysmic variables displays a very pronounced gap between 2 and 3 hours, known as the period gap. It is widely believed that the gap is caused by the temporary cessation of mass transfer when the orbital period has decreased to three hours (possibly related to changes in the magnetic field of the mass-losing star when its interior becomes fully convective) so that the system has no longer the appearance of a cataclysmic variable.