A rare form of cataclysmic binary that appears to consist of a pair of white dwarfs. The few known are extremely hydrogen-deficient and have practically pure helium spectra. There are no outbursts; the variations are like those of a W Ursae Majoris star or ellipsoidal variable, accompanied by rapid flickering. There may be mass transfer to an accretion disk around the primary, and the system may represent an extinct nova. The type star is of 14th magnitude and has the shortest period (18 min) of any known eclipsing binary.