An AM Canum Venaticorum (AM CVn) star is a rare type of cataclysmic binary believed to consist of two white dwarfs in an extremely tight orbit that form an accreting, semidetached system (see close binary). AM CVn stars are helium-rich with no detectable trace of hydrogen, have very short orbital periods – that of the prototype is about 18 minutes – and show permanent super-humps. They are thought to be sources of gravitational waves strong enough to be detectable by LISA, the Laser Interferometry Space Anetenna.