End points of stellar evolution, they are binary systems in which one star is sucking material out of its partner. They revolve around each other very rapidly, typically every few hours.
CVs can also exhibit outbursts on the time scale of weeks to months.
XMM-Newton has been observing one such cataclysmic variable, named OY Car. Looks like a single star but in fact there are two seen here. CVs are composed of two stars, one a very dense and compact star, the other a star similar to our Sun. The less massive star is a red dwarf. Because these two stars are very close together, material is lost from the red dwarf and spirals into the white dwarf.