Named in memory of P. Th. Oosterhoff (1904–1978), professor of astronomy at Leiden University from 1948 until his retirement in 1972, Assistant General Secretary of the IAU during 1951–1952 and General Secretary during 1952–1958. He was well known for his contributions to photographic photometry and for his investigations of Cepheids with double periods. (M 4785) _ _.