The metallicity of the old open cluster NGC 6791

We have observed four red clump stars in the very old, metal-rich open cluster NGC 6791 to derive its metallicity using the high-resolution spectrograph SARG mounted on the Galileo National Telescope (TNG). Using a spectrum synthesis technique, we obtain an average value of [Fe/H] = +0.47 (+/- 0.04, rms 0.08) dex. Our method was tested on mu Leo, a well-studied, metal-rich field giant. We also derive average oxygen and carbon abundances for NGC 6791 from synthesis of [O I] at 6300 angstrom and C-2 at 5086 angstrom, finding O/Fe similar or equal to -0.3 and [C/Fe] similar or equal to 0.2.