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In close binary systems, rotation and tidal forces of the component starsdeform each other and destroy their spherical symmetry. We present new modelsfor low-mass, pre-main sequence stars that include the combined distortioneffects of tidal and rotational forces on the equilibrium configuration ofstars. We investigate the effects of interaction between tides and rotation onthe stellar structure and evolution. The Kippenhahn & Thomas (1970)approximation, along with the Clairaut-Legendre expansion for the gravitationalpotential of a self-gravitating body, is used to take the distortion effectsinto account. We obtained values of internal structure constants for low-mass,pre-main sequence stars from stellar evolutionary models that consider thecombined effects of rotation and tidal forces due to a companion star. We alsoderived a new expression for the rotational inertia of a tidally androtationally distorted star. Our distorted models were successfully used toanalyze the eclipsing binary system EK Cep, reproducing the stellar radii,effective temperature ratio, lithium depletion, rotational velocities, and theapsidal motion rate in the age interval of 15.5-16.7 Myr. In the low-massrange, the assumption that harmonics greater than j=2 can be neglected seemsnot to be fully justified, although it is widely used when analyzing theapsidal motion of binary systems. The non-standard evolutionary tracks arecooler than the standard ones, mainly for low-mass stars. Distorted modelspredict more mass-concentrated stars at the zero-age main-sequence thanstandard models.

Combined effects of tidal and rotational distortions on the equilibrium configuration of low-mass, pre-main sequence stars