PHOTOMETRIC MONITORING OF OPEN CLUSTERS. II. A NEW M DWARF ECLIPSING BINARY SYSTEM IN THE OPEN CLUSTER NGC 1647

We have discovered a new M dwarf eclipsing binary system in the open cluster NGC 1647. Unlike field binaries, accurate age and metallicity estimates for the system are potentially available through an analysis of the brighter cluster members. This information will ultimately allow a detailed and robust comparison with the appropriate stellar evolution models. Analysis of our spectroscopic radial velocity data and photometric light-curve data gives an orbital period of 0.618790 (plus_minus) 0.000005 days and the following properties for the individual stellar components: M1 = 0.47 (plus_minus) 0.05 M, M2 = 0.19 (plus_minus) 0.02 M, R1 = 0.57 (plus_minus) 0.02 R., R2 = 0.21 (plus_minus) 0.01 R., Teff1 = 3320 (plus_minus) 150 K, and Teff2 = 2910 (plus_minus) 150 K. The small mass ratio and low secondary mass provide an unprecedented opportunity to test stellar models. Adopting an age of 150 Myr, consistent with the cluster turn-off, and testing a range of metallicities, covering plausible values for local open clusters, we find that none of the models are consistent with all the properties of both M dwarf stars in the eclipsing binary.