Abundances of four open clusters from solar stars

Aims. 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 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. Methods. We used UVES high-resolution, high signal-to-noise (S/N) ratio spectra and performed a differential analysis with respect to the sun, by measuring equivalent widths and assuming LTE. Results. The most surprising result is a measurement of 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