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We showed in a preceding paper based on an analysis of the observed rates of apsidal motion that synchronization in early-type eclipsing binaries continues on the main sequence, and the observed synchronization times, t syn , agree with the Zahn's theory and are inconsistent with the shorter time-scale proposed by Tassoul. It follows from this that circularization in early-type binaries must also proceed in accordance with the Zahn's theory because the circularization times, t circ , in both theories are rather tightly related to t syn via relation t circ ≈ αt syn , where α is the orbital-to-axial momentum ratio.To further investigate this problem, we compile a catalogue of 101 eclipsing binaries with early-type main-sequence components (M 1,2 > 1.6 M ). We determine the ages, t, and circularization time-scales, t circ , for all these systems in terms of the two competing theories by comparing observational data with modern models of stellar evolution of Claret and atmospheric models of Kurucz. We compute t circ with the allowance for the evolutionary variations of the physical parameters of the components and, for the first time in such studies, also take into account the variations of the orbital parameters (P , a, e) in the process of circularization subject to the conservation of the total angular momentum.The results of these computations show that the mechanism of orbital circularization in earlytype close binary systems (CBSs) suggested by Tassoul is, like in the case of synchronization, inconsistent with observational data. At the same time, the Zahn's mechanism, which is based on the dissipation of the energy of dynamic tides in the upper layers of the envelopes of CBSs components due to non-adiabaticity of these layers, agrees satisfactorily with observations.

The numerical computations of the circularization of close binary systems at the pre-mainsequence (pre-MS) stage of evolution (reported by Zahn and Bouchet in their now well-known paper) are repeated for an extended stellar mass interval of M = 0.1-2.7 M , based on modern evolutionary pre-MS stellar models and on the modified Zahn theory. The new results mostly corroborate those obtained by Zahn and Bouchet in the mass interval M = 0.5-1.25 M covered by their work. To compare the theoretically expected results with observations, a catalogue is compiled of 70 eclipsing binaries with eccentric orbits and bona fide photometric elements and masses. The pre-MS lifetimes of all these systems are computed as fractions of the circularization time-scale t PMS /t circ in terms of the Zahn theory and assuming the conservation of angular momentum L. All systems of this catalogue fill almost uniformly the entire domain of the e obs -t PMS /t circ diagram independently of t PMS /t circ [i.e. the (t PMS /t circ ) cr = 4 threshold does not separate circular and eccentric systems in this diagram]. This result shows that the computations of the orbital circularization at the pre-MS stage based on the concept of the conservation of L have nothing to do with the actually observed distribution of the eccentricities of close binary systems. The results obtained lead us to the following conclusions. (i) Binaries appear to cross the Hayashi phase in a more detached state than we observe them on the MS, and therefore they are unaffected by tidal circularization at this stage. (ii) Binaries must lose an appreciable fraction of their angular momentum while evolving from the Hayashi phase to the zero-age main sequence. This loss may be a result of the dynamic interaction with the parent disc and other components of the multiple system.