Ultraviolet spectral morphology of the O stars. III - The ON and OC stars

Key regions in the 1200-1900-A spectra of two ON dwarfs and four ON/OC supergiants are illustrated and discussed, using high-resolution data from the International Ultraviolet Explorer archives. Marked anomalies are displayed by both stellar wind and photospheric C and N features, relative to the consistent framework provided by normal ultraviolet O-type spectra. Hence these peculiar objects provide a further example of close correspondence between optical spectral classifications and ultraviolet spectral features, as well as between photospheric and wind properties. The opposite departures from normal behavior of the N-versus-C features strongly suggest an origin in abundance effects.