We present the abundance measurements of several elements (Fe, Ca, Na, Ni, Ti, Al, Cr, Si) for 20 solar--type stars belonging to four Galactic open clusters: NGC 3680, IC 4651, Praesepe, and M 67. Oxygen abundances were in addition measured for most of the stars in each cluster apart from IC 4651. For NGC 3680 accurate abundance determinations using high--resolution spectra covering a large spectral domain are computed for the first time. We used UVES high--resolution, high signal to noise ratio spectra and performed a differential analysis with respect to the sun, by measuring equivalent widths and assuming LTE. The most surprising result is a significant supersolar metallicity for Praesepe ([Fe/H]=0.27+-0.10). As for the other clusters, we confirm a supersolar metallicity for IC 4651 ([Fe/H]=0.12+-0.05), a solar metallicity for M 67 ([Fe/H]=0.03+-0.04) and a slight subsolar metallicity for NGC 3680 ([Fe/H]=-0.04+-0.03). We find that the abundance ratios of almost all elements are solar, with the notable exception of oxygen in NGC 3680 and Praesepe, supersolar in the former cluster ([O/Fe]=0.2+-0.05) and as low as [O/Fe]=-0.4+-0.1 in the latter. Observations of several objects per cluster is required to obtain robust results, especially for those elements with a limited number of suitable lines.
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