CNO abundances in 28 open clusters

The results were compared with the theoretical predictions and used for the analysis of the [C/N] relations with age. The origin of the two Theia 1214 stars was investigated. Precise age dating requires separate age calibrations of [C/N] ratios for the first-ascent giants of the lower part of the red giant branch and for the red clump stars to account for the additional abundance alterations during the post-RGB luminosity bump evolution.

In the first-ascent giant stars with larger turn-off masses, the observed C/N ratios are slightly higher than those predicted by standard models, and the obtained [C/N] versus age relation is flatter in the young age regime than in previous studies. We doubt that two stars investigated in the Theia 1214 association belong to the tail of NGC 752, most probably, they are field stars.