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Context. Determining carbon and nitrogen abundances in red giants provides useful diagnostics to test mixing processes in stellar atmospheres.
Aims. Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters Collinder 261 and Melotte 66 and to compare the results with predictions of theoretical models.
Methods. High-resolution spectra were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C₂ Swan (0, 1) band head at 5635.5Å. The wavelength interval 7940-8130Å, which contains CN features, was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [OI] line at 6300Å.
Results. The mean values of the elemental abundances in Collinder 261, as determined from seven stars, are: [C/Fe]=-0.23±0.02 (s.d.), [N/Fe]=0.18±0.09, [O/Fe]=-0.03±0.07. The mean ¹²C/¹³C ratio is 11±2, considering four red clump stars and 18 for one star above the clump. The mean C/N ratios are 1.60±0.30 and 1.74, respectively. For the five stars in Melotte 66 we obtained: [C/Fe]=-0.21±0.07 (s.d.), [N/Fe]=0.17±0.07, [O/Fe]=0.16±0.04. The ¹²C/¹³C and C/N ratios are 8±2 and 1.67±0.21, respectively.
Conclusions. The ¹²C/¹³C and C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolution models. The mean values of ¹²C/¹³C ratios in Collinder 261 and Melotte 66 agree well with models of thermohaline-induced extra-mixing for the corresponding stellar turn-off masses of about 1.1-1.2M_☉. The mean C/N ratios are not decreased as much as predicted by the model in which the thermohaline- and rotation-induced extra-mixing act together.