CRITICAL TESTS OF STELLAR EVOLUTION IN OPEN CLUSTERS .2. MEMBERSHIP, DUPLICITY, AND STELLAR AND DYNAMICAL EVOLUTION IN NGC-3680

Based on new, accurate photometry. radial velocities, and proper motio ns for the intermediate-age open cluster NGC 3680, we identify individ ual single and binary cluster members and field stars in the colour-ma gnitude diagram (CMD). This basic step turns out to be crucial for a p roper understanding of the cluster CMD: similar to 60% of the stars ar e found to be field stars, and over 50% of the cluster stars are binar ies. No bona Side cluster star is found more than 1.(m)5 below the tur noff, and cluster stars below 1.4 M. are only found in binary systems. The total present mass of NGC 3680 is similar to 100M., excluding any as yet unseen stellar remnants, and its half-mass radius is 3.3' (1.2 pc). Comparison with plausible IMFs indicates that only similar to 3% Of the original stars and less than or similar to 10% of the mass now survive, similar to 30% of the initial mass being in the form of mass ive stars that have now completed their evolution, and similar to 60% in low-mass stars which may now be located in a distant cluster halo o r perhaps have been lost entirely. The single main-sequence cluster me mbers form an extremely tight sequence in the CMD, with E(b-y) = 0.034 and [Fe/H] = +0.11. A direct fit to the Hyades main sequence yields ( m-M)(0) = 10.5+/-0.2 for NGC 3680. Isochrones from several stellar mod els have been fit to the cluster sequence. When based on consistent uv by colour transformations and the above cluster parameters, these fits are very stable and show that standard models are not acceptable for stars with the turnoff mass of NGC 3680. Overshooting models perform m uch better, but further refinement of the overshooting formalism seems to be needed. The age derived for NGC 3680 is 1.45+/-0.3 Gyr. The lim iting factor in a precise comparison of theory and observations is now the transformation from theoretical to observed parameters, particula rly (broad-band) colours.