The OBn and OBc stars.

A review of present knowledge concerning nitrogen and carbon anomalies in absorption-line OB spectra is presented, from the observational point of view. The procedure whereby such anomalies are discovered and described is explained, and its capabilities as well as limitations are discussed. A list of all presently known OBN and OBC stars is given, which includes several newly discovered examples. In addition, the problem of more moderate CNO anomalies in OB spectra is examined; an important conclusion is that all of the 09.5-BO. 7 supergiants in the Orion Belt and in NGC 6231 (the nucleus of Scorpius OBi) are systematically nitrogen-deficient, relative to the majority of supergiants with the same spectral types. Since the members of these associations have often been used as standards, an extensive list of O9-B 1 supergiants with normal CNO spectra is also given. Finally, the significance and possible origins of these anomalies are discussed. Subject headings: clusters: associations - stars: abundances - stars: early-type - stars: spectral classification