  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 Msun, and to compare them with predictions of
theoretical models. 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 A. The wavelength interval 7940-8130 A with
strong CN features was analysed to determine nitrogen abundances and carbon
isotope ratios. The oxygen abundances were determined from the [Oi] line at
6300 A. 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. 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.
