An AM CVn star, or AM Canum Venaticorum star, is a type of cataclysmic variable star.
AM CVn stars differ from most other cataclysmic variables (CVs) in the lack of hydrogen lines from their spectra.
AM CVn stars with a white-dwarf donor can be formed when a binary consisting of a white dwarf and a low-mass giant evolve through a common-envelope (CE) phase.
AM CVn stars with a helium-star donor are formed in a similar way, but in this case the giant that causes the common envelope is more massive and produces a helium star rather than a second white dwarf.
SDSS J012940.05+384210.4 is thus confirmed to be a sixth member of the outbursting group of AM CVn stars. The observed rebrightening episodes are similar to those seen during the 2003 June outburst of the AM CVn star V803 Cen.
The AM CVn stars are a class of cataclysmic variables comprising two interacting white dwarfs.
Very few confirmed AM CVn stars are known: only about two dozen are suspected at the time of writing.
Apparently RX J0806.3+1527 has been retreating from its minimum period for a shorter time than any of the other known AM CVn stars
, whose orbital periods range from 10 to 50 minutes.