High-precision study of proper motions and membership of 924 stars in the central region of Praesepe.

High-precision proper motions for 257 and 296 stars in a 90'x90' region centered on BD+20 2170 were obtained from seven plates taken with the 40cm refractor (f=6895mm) at Zo-Se station of Shanghai Observatory with six AC plates and the stellar positions in Russell's (1976) catalogue, respectively. Combining with the data given by Klein-Wassink (1927), Jones (1971), Artyukhina (1971), and Jones & Cudworth (1983), these proper motions give a sample of 924 stars in the region mentioned above. With membership probabilities estimated by an improved maximum likelihood method, a very good sample of 198 members is obtained with proper motion accuracies ranging from +/-0.2 to +/-5.0mas/yr, of which 60 per cent are better than +/-1.0mas/yr, and the completeness nearly down to B=15.5.