Absolute dimensions of eclipsing binaries. XVIII. The Cepheus OB 3 member CW Cephei.

Definitive physical parameters for the components of the eclipsing binary CW Cep (Sp = B0.5 V, V 7.8, P = 2^d.7), member of the Cep OB 3 association, have been determined. Analysis of new uvby light curves combined with existing radial velocity curves (Popper 1974) lead to radii of 5.5±0.1 R_sun (star A) and 5.0±0.1 R_sun (star B) and masses of 11.8±0.1 M_sun and 11.1±0.1 M_sun. This places CW Cep among the very few detached massive systems for which accurate dimensions are available. The larger and more massive star is also the hotter one, although the effective temperatures are nearly equal (28300 K and 27700 K). The new accurate photometry indicates the presence of a faint (red) third light/body. The orbit of CW Cep is slightly eccentric (e =0.0293) and apsidal motion with a period of 45.6±0.4 yr is present.

A satisfactory fit to CW Cep is provided by recent theoretical stellar models, both standard models (Claret & Giménez 1989) and models including stellar-wind mass loss and convective-core overshooting (Maeder & Meynet 1987). CW Cep is located in the lower part of the main sequence band at an age of (10±1)10⁶ yr. A small discrepancy between observed and theoretical density concentration coefficients exists, and rotational velocities significantly higher than those corresponding to synchronous rotation have been reported, whereas current theories (Press et al. 1975) predict that synchronization should have occurred very rapidly.

CW Cep belongs to the older subgroup of the Cep OB 3 association. The age determined for the binary, based on accurate masses and radii rather than scale dependent radiative parameters, clearly supports previous estimates of the nuclear age of the subgroup. Kinematic methods yield a much lower expansion age, a long-standing problem known from several associations.