Light element abundances in the young open clusters NGC 3293, NGC 4755 and NGC 6231: Tracers of stellar evolution

The abundances of He (LTE), C, N, and O (NLTE) were derived for 21 B stars in three young open clusters. Almost all the stars show subsolar CNO abundances. However, the mean oxygen abundance for each programme cluster appears to be in marginal agreement with the most recent revisions of the solar value. After consideration of the CN abundances in this sample, there is no clear evidence of internal mixing. Only three stars among the non-supergiants seem to show a nitrogen enhancement. Two of them have a fairly low projected equatorial velocity (admittedly, they may be rapid rotators seen pole-on); the third one is a definite fast rotator. In the lower gravity stars (three stars in this sample with log g < 3.0) some kind of mixing has apparently occurred. The supergiants do not differ significantly from the other programme stars in their respective helium contents. The mean helium abundance for each cluster is close to the standard value, (He/H) ≈ 11.0.