An Etymological Dictionary of Astronomy and Astrophysics

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.
For a review see G. Nelemans 2005, astro-ph/0409676.

See also: AM, letters of alphabet used in variable star designations; CVn, abbreviation of → Canes Venatici; → star.