Evolution of lithium beyond the solar age: A Li survey of the old open cluster NGC 188

Radial-velocity and BV CCD observations of 29 red-giant candidates in the central part of the intermediate-age open cluster NGC 1817 have confirmed the membership of 21 stars. In addition, we have used radial velocities and new UBVR photoelectric photometry to investigate the membership of 59 new candidates located in an outer corona surrounding NGC 1817, identified from the POSS plates. We have confirmed 18 new members out to a distance of 27 ʹ from the centre, thus doubling the previous cluster radius. Ten definite spectroscopic-binary members have been discovered, corresponding to a binary frequency of 25.6% and four orbits have been determined so far. Three additional stars have been identified as possible spectroscopic-binary members. We derive a cluster mean velocity of +65.33 +,- 0.09 (0.52 s.e.) km s -1 based on 29 members with no detected velocity variations plus the 3 binary members with orbital solutions. The radial distribution of the 39 red-giant members shows a strong concentration of spectroscopic binaries in the inner part of the cluster, similar to that observed in M 67, although NGC 1817 is much younger.