Chemical composition of evolved stars in the open cluster NGC 2506

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 Op- tical Telescope to investigate abundances of up to 25 chemi- cal elements. Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5 ˚ A. The wavelength interval 7980-8130 ˚ A 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 ˚ A. 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 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 of1:70:2 and244 in the giants and to the values of 1:40:2 and164 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.