Z-LAC - ORBITAL CIRCULARIZATION IN SUPERGIANTS

We have obtained new radial velocities for the binary Cepheid Z Lac. T hese data have been combined with new velocities by Imbert (1996) and other data in the literature to derive an orbit. The orbital period is close to a year, making Z Lac the Cepheid with the shortest orbital p eriod known. It is also the only Cepheid with an orbital eccentricity indistinguishable from zero, Comparison between the projected semi-maj or axis and the radii of 5 and 7 M. stars at the tip of the red giant branch [from the evolutionary tracks of Schaller et al. (1992)] indica tes that the orbit was probably circularized during the red giant phas e. (C) 1996 American Astronomical Society.