A hot subdwarf is a helium burning star, with a helium core mass of about 0.50 solar mass, that is covered with a very thin hydrogen shell of about 0.02 solar mass. Hot subdwarfs are immediate progenitors of white dwarfs. There are several types, recognized by their optical spectra, as listed in the table. Types sdB and sdOB are thought to be extremely blue horizontal branch stars that evolved (like HBB stars) from red giants. They form a relatively narrow sequence at the blue end of the horizontal branch (HB), and are therefore often referred to as extended horizontal branch stars (EHB). They differ from normal HB stars in that they don't evolve to the asymptotic giant branch. By contrast, sdO stars are in a post asymptotic giant branch phase of evolution.