Hot subluminous stars
Hot subluminous stars are ubiquitous in all Galactic populations and provide most of the UV radiation from early-type galaxies. In the Hertzsprung-Russell-diagram the hot subluminous stars are located between luminous blue main-sequence stars and the white dwarf sequence which suggests that they are evolved stars burning helium in their core. Their high temperatures imply that the stars have lost almost their entire hydrogen envelope, having evolved from red giants by stripping their envelope via mass transfer to a close companions. White dwarfs as well as low-mass main-sequence stars have been discovered to orbit on orbits as tight as the solar radius. We identified some companions to be of substellar mass (brown dwarf).
Progenitors of type Ia supernovae
Hot subdwarfs with white dwarf companions are excellent candidates as progenitors for thermonuclear supernovae. Mass transfer to the white dwarf may drive its mass to a critical amount. which leads to the thermonuclear explosion of its core, occasionally triggered by a helium explosion in the envelope. Alternatively, the critical mass could be reached by a merger of double white dwarfs.
Hot subluminous stars as testbeds of stellar evolution
The chemical composition of hot subluminous stars
Extreme Horizontal branches of globular clusters