1 Introduction

The distribution of orbital eccentricities versus orbital period has now been studied for binaries of several coeval samples. In all cases a common feature is observed, i.e., binaries with orbital period shorter than a critical period, called the circularization period ( $P_{\rm circ}$), have circular orbits, while the wide binaries have significant orbital eccentricities. This common feature of an $e-\log P$ diagram is usually interpreted as evidence that tidal interactions are driving the orbital evolution. Two distinct mechanisms are usually invoked to explain orbital circularization. In the first one, the tidal-torque mechanism, viscosity will make tides lag (or precede) the line joining the centers of the two components, and this misalignment will generate a torque which will be responsible for the circularization of the orbit (Zahn 1966; Zahn 1977). A key issue in describing tidal-torque interaction is the treatment of the viscosity which has been subject to much discussion. It dictates how strong the dissipative mechanisms are. In terms of orbital period, for a given amount of time, the stronger the dissipative mechanisms are, the longer will be the longest orbital period presenting circular orbit (i.e., the circularization period as defined by Duquennoy et al. 1992). Zahn (1977) (see also Zahn 1989; Goldman & Mazeh 1991) identifies the turbulent friction and the radiative damping as being the main source of viscosity in late- and early-type stars, respectively.

The second mechanism, hydrodynamical mechanism, was suggested by Tassoul in a series of papers (see Tassoul 1995; Tassoul 2000 and references therein). It involves large-scale hydrodynamical flows within the non-synchronous tidally distorted component. Their existence is always required to satisfy the stress-free condition in the star surface. These flows will replace high angular velocity fluid by low angular velocity fluid, braking the star.

Regardless of the mechanism believed to act, the time required to circularize orbits, $t_{\rm circ}$, up to a critical period $P_{\rm circ}$will have the same functional relation $t_{\rm circ} \propto P^{\gamma}_{\rm circ}$. The choice of the mechanism (and dissipation prescription for the tidal-torque mechanism) is reflected by the value of $\gamma$. The relative sensitivity of $P_{\rm circ}$ to stellar age motivated Mathieu & Mazeh (1988) to propose that the observed circularization period for a coeval sample of binaries could in fact be used as a clock to determine the age of galactic clusters. A different approach was proposed by Zahn & Bouchet (1989). These authors, following the suggestion of Mayor & Mermilliod (1984) that the circularization of the orbit would occur very fast during the PMS phase, integrated the full set of equations governing the tidal evolution for binary systems with masses ranging from 0.5 to 1.25 $M_{\odot}$ from the birth-line up to 10¹⁰ years. They found that most of the orbital circularization occurs during the pre-main sequence phase, primarily near the stellar birth-line. According to their results, the circularization period set during the PMS phase is about 7-8.5 days, depending on stellar mass. In addition, as no further significant circularization occurs on the main-sequence, they concluded that the circularization period for all binary populations (age < 10¹⁰ years) should be equal to that established during the PMS phase. Therefore, a determination of the circularization period for binary populations of different ages is needed to enable us to test both the different dissipation prescriptions and the effectiveness of the PMS tidal circularization.

While the circularization period is determined for some young cluster binary populations and for the two older binary populations of M 67 and of the Galactic Halo, for the PMS binaries Mathieu (1994) counted only 25 spectroscopic binaries with known orbital elements. These numbers have been increasing in the last years thanks to many WTTS found by the ROSAT X-ray all-sky survey in the nearby star-forming regions. The preliminary results of on-going spectroscopic monitoring surveys on these stars (Covino et al. 2001a) reveal that many of them are in fact spectroscopic binaries.

In this paper we present a new PMS binary system with orbital parameters that make it a crucial system for testing theory. We confront both Mathieu & Mazeh's and Zahn & Bouchet's hypothesis to the new available data on PMS spectroscopic binaries to review the problem of tidal interactions.