Astron. Astrophys. 324, 91-96 (1997)

1. Introduction

Within a series of papers devoted to the study of red giants in open clusters by means of CORAVEL radial-velocity data and new photoelectric photometry, we discuss in the following the membership and duplicity of 24 red giants in the field of the intermediate-age cluster Mel 71 = C0735-119 (, , B1950). Knowledge of the detailed distribution of the red giant stars in the colour-magnitude diagram is important for comparisons with theoretical isochrones in order to better delineate the precise evolutionary path of the red giants. The determination of the membership and binary character of the individual red giants enables us to define the single-star locus. Our long-term goal is to study the general distribution of orbital elements of cluster red-giant spectroscopic binaries, and a large effort is being made to monitor more than 150 stars with variable radial velocities. One of the main interests of this programme is the possibility of determining the distribution of orbital elements for spectroscopic binaries with different masses of the primary by combining data for red giants and red dwarfs in open clusters and field dwarfs. For example, any difference in the mass ratio distribution, f() or f(), in function of the primary mass will be an interesting test of the various binary star-formation scenarios. Preliminary results were published by Mermilliod & Mayor (1992, 1996), and results for five clusters with ages similar to that of Mel 71 were given in Paper III of this series (Mermilliod & Mayor 1990).

Although Mel 71 is not very faint, it had not been studied before Hassan (1976) published UBV photographic photometry for 230 stars. The colour-magnitude diagram shows that the cluster is a little older than the Hyades and shows prominent red giant clump. Later, Pound & Janes (1986) also published UBV photographic photometry for 631 stars, mostly fainter than V = 14. Geisler et al. (1992) observed 16 red giants in the Washington photometric system and derived a low metallicity = -0.57 0.18, assuming = 0.10 (Pound & Janes 1986). The presence of numerous red giants in the field of Mel 71 makes it an interesting cluster in which to study the red giant population.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

helpdesk.link@springer.de