Hot subluminous stars
  Hot subluminous stars of spectral type B and O are core helium-burning stars
at the blue end of the horizontal branch or have evolved even beyond that
stage. Strikingly, the distribution in the Hertzsprung-Russell diagram of
He-rich vs. He-poor hot subdwarf stars of the globular clusters omega Cen and
NGC~2808 differ from that of their field counterparts. The metal-abundance
patterns of hot subdwarfs are typically characterized by strong deficiencies of
some lighter elements as well as large enrichments of heavy elements. A large
fraction of sdB stars are found in close binaries with white dwarf or very
low-mass main sequence companions, which must have gone through a
common-envelope phase of evolution.They provide a clean-cut laboratory to study
this important but yet purely understood phase of stellar evolution. Substellar
companions to sdB stars have also been found. For HW~Vir systems the companion
mass distribution extends from the stellar into the brown dwarf regime. A giant
planet to the pulsator V391 Peg was the first discovery of a planet that
survived the red giant evolution of its host star. Several types of pulsating
star have been discovered among hot subdwarf stars, the most common are the
gravity-mode sdB pulsators (V1093 Her) and their hotter siblings, the p-mode
pulsating V361 Hya stars. Another class of multi-periodic pulsating hot
subdwarfs has been found in the globular cluster omega Cen that is unmatched by
any field star. The masses of hot subdwarf stars are the key to understand the
stars' evolution. A few pulsating sdB stars in eclipsing binaries have been
found that allow mass determination. The results are in good agreement with
predictions from binary population synthesis. New classes of binaries, hosting
extremely low mass (ELM) white dwarfs (M<0.3 Msun), have recently been
discovered, filling a gap in the mosaic of binary stellar evolution.
(abbreviated)
