1994A&A...290..868S

We present high-resolution, high signal-to-noise spectroscopic observations of the Galactic late-O supergiants HD154811 (OC9.7Iab), HD154368 (O9.5Iab), and HD123008 (ON9.5Iab). The spectra of these stars exemplify the carbon and nitrogen line-strength anomalies characteristic of the ONC supergiants. A hydrogen/helium non-LTE model-atmosphere analysis is carried out using extensive model atoms. Helium abundances (by number) of 0.09, 0.13, and 0.17 are obtained for the OC, normal O, and ON stars respectively. These results are consistent with Walborn's hypothesis that OC supergiants have a normal (i.e., cosmic) helium abundance whereas the morphologically-normal O supergiants and, to a greater extent, the ON supergiants exhibit the products of nucleosynthetic processing at their surfaces. We note that the diagnostic line HeIIλ4200 is probably contaminated by a NIII line in late-O supergiants.