Determination of proper motions and membership of the open clusters NGC 1817 and NGC 1807

Relative proper motions, their corresponding observed errors and membership probabilities of 722 stars in the old open cluster NGC 1817 region are determined from PDS measurements of 12 plate pairs. The plates have the maximum epoch difference of 81 years and were taken with the double astrograph at Zǒ-Sè station of Shanghai Observatory, which has an aperture of 40 cm, a focal length of 6.9 m and a plate scale of 30''/mm. The average proper motion accuracy is 1.6 mas/yr. These proper motions are used to determine the membership probabilities of stars in the region. The existence of two clusters in the region (NGC 1817, NGC 1807) is discussed. The number of stars with membership probability higher than 0.7 is 416 for NGC 1817 and 14 for NGC 1807. Table 5 is only available in electronic form from CDS via anonymous ftp 130.79.128.5, and by e-mail request to lbalague@pchpc3.am.ub.es