  The validity of the classical formula for the rate of secular apsidal motion
in close binaries is investigated for a sequence of models of a 5 solar mass
star ranging from the last stages of the C12 -> N14 reaction to the phase where
hydrogen is exhausted in the core. For binaries with short orbital periods, the
apsidal-motion rates predicted by the classical formula deviate from the rates
determined within the framework of the theory of dynamic tides due to the
effects of the compressibility of the stellar fluid and due to resonances of
dynamic tides with free oscillation modes of the component stars (Smeyers &
Willems 2001). As the star evolves on the main sequence, the deviations caused
by the compressibility of the stellar fluid increase with increasing radius of
the star. The additional deviations caused by the resonances are largest near
the end of the core-hydrogen burning phase. Both of these deviations increase
with increasing values of the orbital eccentricity.
