In the framework of a project aiming at deriving in a homogeneous way the properties (age, distance, reddening and detailed chemical abundances) of a large sample of old open clusters, we present here the metal abundance and the abundance ratios of light (C, N, O, Na, Mg, Al, Si, Ca, Ti) and heavier (Cr, Mn, Ni, Ba, Eu) elements in the galactic open clusters NGC 6253 and NGC 6791. We performed spectrum synthesis of selected lines on high resolution spectra of four red clump stars in NGC 6253, taken with the UVES and FEROS spectrographs. We also determined abundances of the same elements for four red clump stars in NGC 6791, observed with SARG, for which we had derived the atmospheric parameters and the iron, carbon and oxygen abundances in a previous paper (Gratton et al. 2006). The average metallicity of NGC 6253 is [Fe/H]=+0.46 (rms = 0.03 dex, systematic error = 0.08 dex), obtained by extensive spectral synthesis of Fe lines. This intermediate age cluster closely resembles the old open cluster NGC 6791, as far as the chemical composition is concerned. C, N, O do not show any significant abundance scatter; they are underabundant with respect to the solar values both in NGC 6253 and NGC 6791. We also find no evident star-to-star scatter in any of the elements measured in both clusters, with the possible exception of Na in NGC 6791. The two clusters show very similar abundances, except for Mg, overabundant in NGC 6791 and not in NGC 6253. Both have solar scaled alpha-elements abundances. We have compared our abundance ratios with literature values for disk giants and dwarfs and bulge giants, finding a general good agreement with the run of elemental ratios with [Fe/H] of disk objects.
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