In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.
They show a broad continuum corresponding to hot stars with complex absorption or emission lines.
In the high state, stars show brightness variations of a few tenths of a magnitude with multiple short periods, less than or around 20 minutes.
The spectra show absorption lines mainly of helium, and the high state is so named as it is similar to a permanent outburst.
In addition to the three standard types of variability, extreme short period (< 12 minutes) stars show only tiny very rapid brightness variations.
[3] It is possible for AM CVn systems to show eclipses, but this is rare due to the tiny sizes of the two component stars.
Temperatures over 100,000 K have been proposed for some stars (e.g. ES Ceti), possibly with direct impact accretion without a disk.
[2] In some cases a donor white dwarf may have a comparable mass to the accretor although it is inevitably somewhat lower even when the system first forms.
As the donor star is stripped it expands adiabatically (or close to it), cooling to only 10,000–20,000 K. Therefore, the donor stars in AM CVn systems are effectively invisible, although there is the possibility of detecting a brown dwarf or planet sized object orbiting a white dwarf once the accretion process has stopped.
[1] 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.
When the orbital period has shrunk to about 5 minutes, the less-massive (and the larger) of the two white dwarfs will fill its Roche lobe and start mass transfer to its companion.
If the donor starts mass transfer too long after the TAMS, the mass-transfer rate will be high and the orbit will diverge.
[1][2] Before settling into an AM CVn state, binary systems may undergo several helium nova eruptions, of which V445 Puppis is a possible example.