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Aims. Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters NGC 2324, NGC 2477, and NGC 3960, which have turn-off masses of about 2 M⊙, and to compare them with predictions of theoretical models. Methods. High-resolution spectra were analysed using a differential synthetic spectrum method. Abundances of carbon were derived using the C2 Swan (0, 1) band heads at 5135 and 5635.5 Å. The wavelength interval 7940-8130 Å with strong CN features was analysed to determine nitrogen abundances and carbon isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 Å. Results. The mean values of the CNO abundances are [C/Fe] = -0:35 ± 0:06 (s.d.), [N/Fe] = 0:28 ± 0:05, and [O/Fe] = -0:02 ± 0:10 in seven stars of NGC 2324; [C/Fe] = -0:26 ± 0:02, [N/Fe] = 0:39 ± 0:04, and [O/Fe] = -0:11 ± 0:06 in six stars of NGC 2477; and [C/Fe] = -0:39 ± 0:04, [N/Fe] = 0:32 ± 0:05, and [O/Fe] = -0:19 ± 0:06 in six stars of NGC 3960. The mean C/N ratio is equal to 0:92 ± 0:12, 0:91 ± 0:09, and 0:80 ± 0:13, respectively. The mean 12C/13C ratio is equal to 21 ± 1, 20 ± 1, and 16 ± 4, respectively. The 12C/13C and C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolution models. Conclusions. The mean values of the 12C/13C and C/N ratios in NGC 2324 and NGC 2477 agree well with the first dredge-up and thermohaline-induced extra-mixing models, which are similar for intermediate turn-off mass stars. The 12C/13C ratios in the investigated clump stars of NGC 3960 span from 10 to 20. The mean carbon isotope and C/N ratios in NGC 3960 are close to predictions of the model in which the thermohaline- and rotation-induced (if rotation velocity at the zero-age main sequence was 30% of the critical velocity) extra-mixing act together. [ABSTRACT FROM AUTHOR]

High-resolution spectral analysis is performed for the first time in evolved stars of two young open clusters: NGC 4609 and NGC 5316, of about 80 and 100 Myr in age, respectively, and turn-off masses above 5M⊙. Stellar evolution models predict an extra-mixing event in evolved stars, which follows the first dredge-up and happens later on the red giant branch. However, it is still not understood how this process affects stars of different masses. In this study, we determine abundances of the mixing sensitive elements carbon and nitrogen, carbon isotope 12C/13C ratios, as well as 20 other elements produced by different nucleosynthetic processes (O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Y, Zr, Ba, La, Ce, Pr, Nd, and Eu).We compared our results with the latest theoretical models of evolutionary mixing processes. We find that the obtained 12C/13C and C/N ratios and [Na/Fe] agree quite well with the model which takes into account thermohaline- and rotation-induced mixing but within error limits also agree with the standard first dredge-up model. Comparison of oxygen, magnesium, and other a-elements with theoretical models of Galactic chemical evolution revealed that both clusters follow the thin disc a-element trends. Neutron-capture element abundances in NGC 4609 are apparently reflecting its birthplace in the thin disc, while NGC 5316 has marginally higher abundances, which would indicate its birthplace in an environment more enriched with neutron-capture elements. [ABSTRACT FROM AUTHOR]