RED GIANTS IN OPEN CLUSTERS .4. NGC-3680 AND IC-4651

Radial-velocity observations of 35 red giants in the field of the intermediate-age open clusters NGC 3680 and IC 4651 are presented and analysed for membership and duplicity. Five stars in NGC 3680 and one in IC 4651 were identified as non-members, and six and five spectroscopic binaries, respectively, were discovered among the red giant members in the two clusters. Orbits have been determined for seven of the binaries, four of which are still preliminary. The mean cluster velocities of NGC 3680 and IC 4651 are 0.9 +/- 0.2 km s(-1) and -31.0 +/- 0.2 km s(-1) respectively. The morphology of the red giant region in the colour mag nitude diagrams shows a well-populated clump in both clusters. In both clusters, one certain member star is clearly brighter and redder than the clump, while IC 4651 shows, in addition, a probable concentration of stars on the ascending giant branch.