1 Introduction

The similar age of the stars in globular clusters and their distribution in a small region of the sky makes globular clusters vital objects for the study of stellar evolution, especially of the late stages. However for the study of such clusters in crowded star fields at low galactic latitudes it is essential to separate the field stars from the cluster members to be able to work with a "clean'' colour-magnitude diagram (CMD). Proper motions of stars in the region of globular clusters provide a unique possibility of getting membership information of the stars independently from other quantities (see e.g. references in Dauphole et al. 1996). Due to the limiting magnitude of the old plates such studies reach up to now mainly only V = 16.0.

The globular cluster M 71 is located at low galactic latitude ( $b=-5^{\circ}$). Johnson photometry of this cluster was presented in the classical study of Arp & Hartwick (1971), and CCD photometry by Richer & Fahlmann (1988), Heasley & Christian (1991) and Hodder et al. (1992). Most of the CCD studies present deep colour-magnitude diagrams reaching the main sequence far beyond the turnoff point, but give only sparse information of the subgiant branch. In the only existing modern proper motion study Cudworth (1985) combines high accuracy proper motions with photographic B and V photometry to present a CMD based on photometry and membership. However, as noted in the paper and as seen his Fig. 2, the data become statistical uncertain below the horizontal branch at V = 14.5 and give also almost no information on the subgiant branch of M 71.

We have performed CCD two-colour photometry of a field of about 20 $\hbox {$^\prime $ }$ $\times$ 20 $\hbox {$^\prime $ }$ around M 71 with the exception of the stars in the very centre of the cluster. Our data are the base not only for the investigation of the CMD diagram of M 71, but also for a forthcoming study of the structure of the Milky Way at low galactic latitudes. Here we combine our photometric data of the cluster region with relative proper motions, in order to determine a CMD of M 71 covering mainly the subgiant branch.