We present NLTE abundances of CNO for a sample of four O9 stars in the Galaxy , together with new determinations of their stellar parameters , T _ { eff } , \log g , \epsilon ( He ) and microturbulence .
These new analyses take into account the effect of line–blocking in the spectral synthesis with our classical NLTE , plane–parallel and hydrostatic model atmospheres . The sample includes three O9 He normal stars : two dwarfs , HD 214680 and HD 34078 , and one supergiant , HD 209975 , and one fast rotating giant with a preliminary high He overabundance , HD 191423 with \epsilon ( He ) =0.20 . We find first that the consideration of microturbulence in the spectral synthesis for the fast rotator leads to a considerably lower He abundance , \epsilon ( He ) =0.12 . The CNO abundances of the three He normal stars are in good agreement with the values in the literature for Galactic B dwarfs with no evidence of mixing , and show that they all have the same chemical composition .
We also discuss however the possible CNO contamination of the supergiant HD 209975 .
For the fast rotator we find that the abundances show the trend of the CNO contamination : a N overabundance together with C and O depletion .
The N/C and N/O ratios of our stars as a function of their projected rotational velocities are consistent with the predictions of the recent evolutionary models of Meynet & Maeder [ 2000 ] .