Chemical composition of evolved stars in the open cluster M 67

High-resolution spectra of six core helium-burning 'clump' stars and three giants in the open cluster M 67 have been obtained with the SOFIN spectrogr aph on the Nordic Optical Telescope to investigate abundances of up to 25 c hemical elements. Abundances of carbon were studied using the C-2 Swan (0,1 ) band head at 5635.5 Angstrom. The wavelength interval 7980-8130 Angstrom with strong CN features was analysed in order to determine nitrogen abundan ces and C-12/C-13 isotope ratios. The oxygen abundances were determined fro m the [O I] line at 6300 Angstrom. The overall metallicity of the cluster stars was found to be close to solar ([Fe/H]= -0.03 +/- 0.03). Compared with the Sun and other dwarf stars of t he Galactic disk, as well as with dwarf stars of M 67 itself, abundances in the investigated stars suggest that carbon is depleted by about 0.2 dex, n itrogen is enhanced by about 0.2 dex and oxygen is unaltered. Among other m ixing-sensitive chemical elements an overabundance of sodium may be suspect ed. The mean C/N and C-12/C-13 ratios are lowered to the values of 1.7 +/- 0.2 and 24 +/- 4 in the giants and to the values of 1.4 +/- 0.2 and 16 +/- 4 in the clump stars. These results suggest that extra mixing of CN-cycled material to the stellar surface takes place after the He-core flash. Abunda nces of heavy chemical elements in all nine stars were found to be almost i dentical and close to solar.