|AM Canum Venaticorum stars
A binary system that has very short orbital period (less than one hour) and helium dominated spectrum. The prototype, AM Canum Venaticorum, with a period of 17 minutes, was discovered in 1967. AM CVn stars are semidetached binary systems in which accretion is going on. The donor star is hydrogen deficient and the accretor is usually a white dwarf. To fit within their Roche lobes, the donor stars must be dense, suggesting they may be degenerate too. It is at present thought that AM CVn stars represent three possible evolutionary phases in 1) double white dwarf systems, 2) white dwarf and helium star binaries, and 3) cataclysmic variables with evolved donors.