The astrophysics of ultra-compact binaries : a white paper for the astro2010 decadal review

Ultra-compact binaries are objects which have orbital periods shorter than one hour. Both stars must be compact and are typically degenerate and hydrogen deficient. The class includes interacting AM CVn stars, ultra-compact X-ray binaries, detached double white dwarfs, double neutron stars, white dwarf/neutron star binaries and as yet unobserved binaries such as black holes with neutron star or white dwarf companions.