Vol. 551
In section 8. Stellar atmospheres

An MK-like system of spectral classification for hot subdwarfs

The authors present an MK (Morgan-Keenan)-like system of spectral classification for hot subdwarfs. It did not fit in the classical MK system since they are peculiar stars. They find that a three-dimensional spectral type, consisting of a spectral class, a luminosity class, and a helium class, is needed to classify the sdO (subdwarf O) and sdB (subdwarf B) stars. In addition, the helium-strong stars appear to form two parallel spectral sequences: one showing strong lines of ionized carbon and the other with weak or absent carbon lines.