  Context: Homogeneous investigations of red giant stars in open clusters
contribute to studies of internal evolutionary mixing processes inside stars,
which are reflected in abundances of mixing-sensitive chemical elements like
carbon, nitrogen, and sodium, while alpha- and neutron-capture element
abundances are useful in tracing the Galactic chemical evolution. Aims: The
main aim of this study is a comprehensive chemical analysis of red giant stars
in the open cluster IC 4756, including determinations of 12C/13C and C/N
abundance ratios, and comparisons of the results with theoretical models of
stellar and Galactic chemical evolution. Methods: We used a classical
differential model atmosphere method to analyse high-resolution spectra
obtained with the FEROS spectrograph on the 2.2m MPG/ESO Telescope. The carbon,
nitrogen, and oxygen abundances, 12C/13C ratios, and neutron-capture element
abundances were determined using synthetic spectra, and the main atmospheric
parameters and abundances of other chemical elements were determined from
equivalent widths of spectral lines. Results: We have determined abundances of
23 chemical elements for 13 evolved stars and 12C/13C ratios for six stars of
IC 4756. The mean metallicity of this cluster, as determined from nine definite
member stars, is very close to solar - [Fe/H] = -0.02 +/- 0.01. Abundances of
carbon, nitrogen, and sodium exhibit alterations caused by extra-mixing: the
mean 12C/13C ratio is lowered to 19 +/- 1.4, the C/N ratio is lowered to 0.79
+/- 0.05, and the mean [Na/Fe] value, corrected for deviations from the local
thermodynamical equilibrium encountered, is enhanced by 0.14 +/- 0.05 dex. We
compared our results to those by other authors with theoretical models.
