AM Canum Venaticorum star

An AM Canum Venaticorum 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.