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We present new CCD photometry in the b and y colours of the Stromgren system for 310 stars in a 13 ×13  field centered on the intermediate-age open cluster NGC 3680. Careful cross-checks indicate that previously published photometry of NGC 3680 is affected by random and/or systematic errors precluding its use in critical comparison with theoretical isochrone computations. Detailed notes on several individual stars are given. In addition, we present new photoelectric radial-velocity observations of 109 stars obtained with the CORAVEL scanner during the period 1988-1994. These data allow substantially complete identification of member and non-member stars in the field, and of spectroscopic binaries in both groups. Rotational velocities have also been derived for the programme stars, and our velocity variability criteria for stars of all rotations are described. The further astrophysical discussion of the data, including the definition of radial-velocity membership criteria, theoretical isochrone fitting, and the dynamical state of the cluster and the origin of its “bimodal turnoff", will appear in a separate paper (Nordstrom et al. 1996).

Critical tests of stellar evolution in open clusters. I. New photometry and radial velocities for NGC 3680