Carbon, nitrogen and oxygen abundances in G8-K3 giant stars.

High-resolution scans of the forbidden O I 6300 line and of lines of the red CN(2, 0) band, and very-narrow-band-index measurements of the C2 Swan (0, 1) band head have been obtained for approximately 30 G8-K3 III stars; and CNO abundances have been derived from these data. The errors in the resulting CNO abundances are typically 0.1-0.2 dex for C/H and O/H, and 0.2-0.3 dex for N/H. The abundance results verify the general result of Lambert and Ries that there seem to exist a nitrogen enrichment and a carbon deficiency in most giants as compared with main-sequence stars. The C/N ratio is shown to be significantly smaller for the giants than for the sun, with the exception of Intermediate Population II stars. The O/Fe ratio is shown to be greater than that of the sun for the metal-poor stars, while it is close to solar for the rest of the sample. These tendencies are considered as possible consequences of stellar and galactic evolution.