The cluster ages experiment (case). III. Analysis of the eccentric eclipsing binary V32 in the globular cluster NGC 6397

We present spectroscopic and photometric observations of the eclipsing binary V32 located in the central field of the globular cluster NGC 6397. The variable is a single-line spectroscopic binary with an orbital period of 9.8783 days and a large eccentricity of e = 0.32. Its systemic velocity (gamma = 20.7 km s(-1)) and metallicity ([Fe/H]similar to -1.9) are both consistent with cluster membership. The primary component of the binary is located at the top of the main-sequence turnoff on the cluster color - magnitude diagram. Only a shallow primary eclipse is observed in the light curve. Based on stellar models for an age of 12 Gyr and the mass function derived from the radial velocity curve, we estimate the masses to be M-p = 0.79 M circle dot and M-s = 0.23 M circle dot. The light curve of V32 can be reproduced by adopting R-p = 1.569 R circle dot and R-s = 0.236 R circle dot for the radii and i = 85.44 deg for the system inclination. The system geometry precludes observations of the secondary eclipse. The large eccentricity of the orbit is puzzling given that for metal-poor, halo binaries the transition from circular to eccentric orbit occurs at an orbital period of about 20 days. We suppose that the orbit of V32 was modified relatively recently by dynamical interaction with other cluster star(s). An alternative explanation of the observed eccentricity calls for the presence of a third body in the system.