1 Introduction

The open cluster NGC 1817, in Taurus [$\alpha_{1997}$=$\rm 5^h11^m22^s$,$\delta_{1997}=16^\circ41'34''$], is a rich, unstudied and presumably old open cluster (Janes & Adler 1982; Phelps et al. 1994; Janes & Phelps 1994). To date, the only photometric study of NGC 1817 was performed by Harris & Harris (HH 1997) who obtained UBV photometry to a limit V=16.7, of 265 stars in the central area of this cluster. A preliminary photometric study of the region by Purgathofer (1961, 1964) reached only to V=14. Previously, Cuffey (1938) obtained extensive photographic photometry of stars in this area in the blue and red bands to a limiting magnitude of R=14. The location of the cluster at 1800 pc almost directly in the Galactic anticenter direction and 400 pc below the plane [l=186.1, b=-13.1], and its low metallicity, make it an object of special interest for the research of the structure and the chemical evolution of the Galaxy.

Very little is known about the open cluster NGC 1807. Some authors considered it as not physical open cluster (Becker & Fenkart 1971) or as possible member of multiple system with NGC 1817, 2244 and 2252 (Barkhatova 1963). Some of its G and K stars have DDO observations (Yoss et al. 1981; Piatti et al. 1995).

Proper motions of some stars in this region were published by Li (1954). But the accuracy of its proper motions is rather low since the epoch difference of the plates he used was only twelve years. Later on, in 1980's, three pairs of plates of NGC 1817 taken with the 40 cm astrograph at Shanghai Zo-Sè station, were measured manually on the Zeiss Ascorecord measuring machine at Zo-Sè. Relative proper motions in the cluster region of NGC 1817 were obtained from plate measurements, and its membership determination was made (Tian et al. 1983). However, these proper motions are not very accurate because of manual measuring.

In this paper we will, for the first time, determine accurate relative proper motions of 722 stars within a 1${}^\circ$.5 $\times$ 1${}^\circ$.5 area in the NGC 1817 region, from automatic measurements of 24 plates, six of which are newly taken. Through estimating membership probabilities with an improved likelihood method (Tian et al. 1998), the membership determination will lead us to distinguish two separate cluster in the region, with different distribution parameters: NGC 1817 and NGC 1807.