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CO Lac is an important 10th mag eclipsing and double-line spectroscopic binary with P = 1.542 days. This system consists of two nearly identical B8.5 V and B9.5 V stars moving in an eccentric orbit with e = 0.03. CO Lac has relatively deep, narrow eclipses which permit the times of primary and secondary eclipses to be accurately measured, even from early photographic plate material. Analyses of these timings reveal an apsidal motion period of U $= 43.4\pm0.1$ yrs. CO Lac is a vital laboratory for studying the internal structure of stars because of its well-determined apsidal motion rate and also because the physical properties of its components can be accurately determined from its light and radial velocity curves. A significant problem with previous interpretations of CO Lac has been the uncertainty in determining the masses of the component stars. To refine the masses of the system, a new radial velocity study of the star was undertaken during 1993/94 at Mt. Laguna Observatory using a 1M telescope and a CCD detector. A preliminary analysis of the recent spectroscopy indicates that the masses of the B8.5 V and B9.5 V are 3.2M$_{\odot}$ and 2.8M$_{\odot}$, respectively. These masses are considerably lower than earlier values and are in accord with the stars' spectral types.
We will present new analyses of the light and RV curves using the Wilson-Devinney code and give revised values for the masses and radii of the component stars. These values are used to determine the inter- nal structure parameters (k2) from the most recent interior models of Gimenez and Claret. The internal structure parameters will be com- pared to those found from the rate of apsidal motion to check the interior models.
This research is supported by NSF grant AST93-15365.
