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 spectrograph on the Nordic Optical Telescope to investigate abundances of up to 25 chemical elements .
Abundances of carbon were studied using the { C } _ { 2 } Swan ( 0,1 ) band head at 5635.5 Å .
The wavelength interval 7980–8130 Å with strong CN features was analysed in order to determine nitrogen abundances and ^ { 12 } { C } / ^ { 13 } { C } isotope ratios .
The oxygen abundances were determined from the [ O I ] line at 6300 Å .

The overall metallicity of the cluster stars was found to be close to solar ( [ Fe/H ] = -0.03 \pm 0.03 ) .
Compared with the Sun and other dwarf stars of the 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 , nitrogen is enhanced by about 0.2 dex and oxygen is unaltered .
Among other mixing-sensitive chemical elements an overabundance of sodium may be suspected .
The mean C/N and ^ { 12 } { C } / ^ { 13 } { C } ratios are lowered to the values of 1.7 \pm 0.2 and 24 \pm 4 in the giants and to the values of 1.4 \pm 0.2 and 16 \pm 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 .
Abundances of heavy chemical elements in all nine stars were found to be almost identical and close to solar .