First results of a photometric and astrometric study of the globular cluster M 71 (NGC 6838)

¹ Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
² Observatoire de Bordeaux, URA 352 CNRS/INSU, BP. 89, F-33270 Floirac, France

Send offprint request to: M. Geffert

Received: 7 September 1998
Accepted: 7 March 2000

Abstract


We present the first results of a photometric and astrometric investigation of stars in the region of the globular cluster M 71 (NGC 6838). Using wide field CCD observations we determined B and V magnitudes of 4450 stars up to a limiting magnitude of V ≤ 18.5 mag. Relative proper motions were used to derive membership probabilities for stars with around the centre of M 71. Our colour-magnitude diagram (CMD) of 320 probable cluster members reaches down to V = 18. A well defined red giant bump - for the first time seen in M 71 - could be detected in our CMD. From isochrone fitting we find M 71 metal poorer and older than previously assumed. Four faint variable stars were confirmed to be members of M 71. Moreover, we detected 13 blue stragglers among our cluster members.