1 Introduction

There are a number of open clusters in the direction of the Taurus dark clouds. Dreyer (1888) considered that there are three overlapping open clusters: NGC 1746, NGC 1750 and NGC 1758. Besides these, the Hyades cluster is in the foreground, and the Pleiades cluster immerses itself partly into the front edge of the dark clouds. The other three clusters, as background objects, are reddened by different amounts. This group is very close to the anticenter direction of the Galaxy, with Galactic coordinates $l=179^{\circ}, b=-11^{\circ}.$ Their common angular diameter, given by Alter et al. (1970), is $\sim50^\prime$. The positions of these clusters on the celestial sphere are shown in Fig. 1. Whether or not these clusters exist is a matter of dispute. Some authors considered all three clusters as one: NGC 1746 (Alter et al. 1958, 1970; Ruprecht et al. 1981). Cuffey (1937) obtained extensive photographic photometry of stars in this area in the blue and red bands to a limiting magnitude of 13 mag. He named all of his photographed area NGC 1746. The first photoelectric photometry of stars in this area of the Taurus dark clouds was in the Vilnius photometric system (Straizys & Meistas 1980; Meistas & Straizys 1981; Cernis 1987). V magnitudes, color indices, color excesses, interstellar extinction and distances were determined for 116 stars (Straizys et al. 1992), to a limit of $V \simeq 13$. They concluded that NGC 1746 was probably not a cluster, and that the distances of NGC 1750 and NGC 1758 were 510 pc and 680 pc respectively, if the two groupings were real open clusters.

  

Figure 1: Star map from the Tirion et al. (1987) atlas with the clusters NGC 1746, NGC 1750 and NGC 1758 shown

Historically, proper motions have provided a reliable method for determining membership of stars in open clusters. First-epoch proper motion plates of the region of NGC 1750 and NGC 1758 were taken with the double astrograph at the Zo-Sè station of Shanghai Observatory in 1918-1961. We took the second-epoch plates of the same region with the same telescope in the 1980's. These plates are clearly very valuable for astrophysical studies in the cluster region. Proper motions of some stars in this region were published by Li (1954). However, the accuracy of his proper motions is rather low since the epoch difference of the plates he used was only 12 years and the measuring machine was not very good more than 40 years ago.

In the present paper, a new, improved approach for determining membership probabilities is presented. Relative proper motions for 540 stars within a 1$.\!\!^\circ$5 $\times$ 1$.\!\!^\circ$5 area centered on the Taurus dark clouds are determined using plates taken over a period of 68 years, and the two clusters are successfully separated from each other.