Cataclysmic variable

Cataclysmic variable stars are binary stars that consist of two components; a white-dwarf primary, and a mass-transferring secondary. The stars are so close to each other that the gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star, and it is usually less massive than the primary. The infalling matter, which is usually rich in hydrogen, forms in most cases an accretion disk around the white dwarf. Strong UV and X-ray emission is often detected from the accretion disc, powered by the loss of gravitational potential energy from the infalling material. The shortest observed orbit in a hydrogen-rich system is 51 minutes in ZTF J1813+4251.