  Context. Investigations of abundances of carbon and nitrogen in the
atmospheres of evolved stars of open clusters may provide comprehensive
information on chemical composition changes caused by stellar evolution. Aims.
Our main aim is to increase the number of open clusters with determined
carbon-to nitrogen and carbon isotope ratios. Methods. High-resolution spectra
were analysed using a differential model atmosphere method. Abundances of
carbon were derived using the C_2 Swan (0,1) band head at 5635.5 {\AA} (FEROS
spectra) and the C_2 Swan (1,0) band head at 4737 {\AA} (UVES spectra). The
wavelength interval 7980-8130 {\AA}, with strong CN features was analysed to
determine nitrogen abundances and 12^C/13^C isotope ratios. The oxygen
abundances were determined from the [Oi] line at 6300 {\AA}. Results. The
average value of 12^C/13^C isotope ratios of Cr 261 is equal to 18 \pm 2 in
four giants and to 12 \pm 1 in two clump stars; it is equal to 16 \pm 1 in four
clump stars of the open cluster NGC 6253. The mean C/N ratios in Cr 261 and NGC
6253 are equal to 1.67 \pm 0.06 and 1.37 \pm 0.09, respectively. Conclusions.
The 12^C/13^C and C/N values in Cr 261 and NGC 6253 within limits of
uncertainties agree with the theoretical model of thermohaline-induced mixing
as well as with the cool-bottom processing model
