The classification of helium-rich hot subdwarfs C. S. Jeffery, J. S. Drilling, P. M. Harrison, U. Heber and S. Moehler Astron. Astrophys. Suppl. Ser., 125 3 (1997) 501-510 Published online: 15 November 1997 DOI: 10.1051/aas:1997237