1 Introduction

The computation of theoretical light curves for eclipsing binaries requires previous knowledge of astrophysical parameters including the limb-darkening and the gravity-darkening coefficients. It was shown in Popper (1984) that the fit of geometrical or radiative parameters to actual observations by means of synthetic light curve modeling, is not possible for second order effects like those of limb and gravity darkening. It is therefore advisable to adopt these coefficients from realistic theoretical computations instead of trying to derive them from the observations. In fact, small variations in the main geometrical and radiative parameters of the light curve may easily mask the real value of the darkening coefficients or even lead to a wrong combination of physical parameters actually reproducing the observed light variations.

Limb-darkening coefficients are computed using stellar atmosphere models while the gravity-darkening exponent requires some knowledge of the stellar interior. Several papers have been devoted in the last years to treat the effects of limb-darkening (Van Hamme 1993; Díaz-Cordovés et al. 1995; Claret et al. 1995 and references therein). But the currently adopted values of gravity-darkening are still the old ones based on the results by von Zeipel (1924) and Lucy (1967). Most of the theoretical papers dealing with gravity darkening in stars with convective envelopes are based on the work by Lucy and the results are essentially the same. Alternative formulations include Martynov (1973) and Anderson & Shu (1977). Martynov used the Planck law to derive $\beta_1$ as a function of wavelength, while Anderson & Shu argued that $\beta_1$ should be zero since the flux (almost convective) ought to be constant over equipotentials. Hereafter we denote this exponent as $\beta_1$ in order to differentiate it from the radius of gyration $\beta$. In this paper, we present new computations for $\beta_1$ using a method based on interior models which embrace convective and radiative envelopes. Such calculations are presented for the first time as a function of the mass and degree of evolution.

For the computation of the basic stellar evolutionary parameters, we used the models previously computed by Claret 1995 for a representative chemical composition (X=0.70, Z=0.02). They cover a range of stellar masses (1 up to 40 $M_{\odot}$) and ages. The limb-darkening coefficients for the Strömgren, Johnson and R I J H K systems were then computed, as well as the gravity-darkening exponents, for every theoretical track. For the sake of completeness, the tables produced included the most relevant parameters for the study of the dynamical behavior of binary systems, namely, the apsidal motion constants (log k_j, j=1, 2, 3), the moment of inertia and the potential energy. In addition, synthetic colors and absolute V magnitude have been computed. This provides a complete and coherent table of stellar parameters allowing the modeling of light curves and the analysis of binary stars evolution with self-consistent derived values.

The paper is divided in three parts: this short Introduction; Sect. 2, where we present and discuss the stellar models with the corresponding limb-darkening coefficients and colors, and Sect. 3 that is devoted to the gravity-darkening calculations.