CARBON, NITROGEN AND OXYGEN ABUNDANCES IN YELLOW SUPERGIANTS

Carbon, nitrogen and oxygen abundances were measured in a sample of 9 yellow supergiants for which accurate rotational velocities are now av ailable. We obtain [O/Fe] approximate to -0.2, a result consistent wit h other young objects (main sequence B stars, planetary nebulae, H II regions). We also find [C/Fe] approximate to -0.2 for 7 stars, and app roximate to 0.0 for another 2 ones, and [N/Fe] approximate to 0.0 for 6 stars and approximate to +0.2 for the other 3 ones. Stars showing lo w carbon and/or high nitrogen are assumed to show convective mixing ef fects. The present results seem to indicate that, among the low rotati on supergiants located near the rotational discontinuity at F9Ib, a fr action of them shows no mixing, and another fraction does show mixing effects, probably depending on their evolutionary stage.