1 Introduction

Post-T Tauri stars (pTTS) are pre-main sequence objects with masses similar to those of T Tauri stars, but older and so closer to the main sequence. The pTT stars do not show photometric and spectroscopic signatures of youth that characterize the classical T Tauri stars; only some weak remnant of line emission, IR excess and irregular variability are detectable, but at least the presence of the strong Li I 6708 resonance doublet remains as a characteristic of all the real pTTS, since Li destruction through convective mixing continues, in these stars, up to the early stages of their main sequence life. A strong Li I resonance line is also observed in the evolved stars, in post-main sequence phase (see references for example in Pilachowski et al. 2000) but these stars can be easily distinguished from the pTTs through their higher luminosity.

The pTT stars are supposed to be more numerous, about one order of magnitude higher, according to Herbig (1978), than the classical T Tauri stars because their evolutionary phase is longer. The exceptions may be represented by very young regions where classical T Tauri stars are expected to be present, while the more evolved pTTS had not yet the time to be produced and also if discontinuous star formation occurred in some clouds (Herbig 1978).

The detection of the elusive pTTS in the general field was first discussed by Murphy (1969) and carried out by Gahm et al. (1983) who searched for visual binaries with early type primaries and late type secondaries. The idea was that the main sequence lifetime of the high mass component is comparable to the contraction time scale of solar type stars; so if the systems are physical, the late type secondaries should still be contracting to the ZAMS or have recently arrived on it. With this hypothesis Lindroos (1985, 1986) identified 78 such visual systems. In order to distinguish the physical from the optical systems through the position of the stars in the HR diagram, Lindroos (1985) determined the astrophysical parameters by using Strömgren photometry and low resolution spectroscopy. The coherence of the ages determined by using Hejlesen (1980) and Iben & Talbot (1966) isochrones is then discussed by the same author (Lindroos 1986).

Two studies by Pallavicini et al. (1992) [hereafter PPR] and by Martín et al. (1992) [hereafter MMR] were aimed to analyse the spectroscopic properties of a sample of the Lindroos survey in order to identify the physical systems with genuine pTT components. PPR studied the Li I 6708, H$_{\alpha}$ and the Ca II H and K features in 39 secondaries with spectral types later than F2. MMR added radial velocity measurements to the spectroscopic and photometric analysis of their selected sample of 14 stars. These two studies concluded that only for about 1/3 of the analysed systems is the duplicity physical and that the secondaries are really in the the pTT evolutionary phase.

It should be noted that for some stars there are discrepancies on the conclusion reached by PPR and MMR, for example for HD 27638 and HD 127304.

Distances derived from the Hipparcos (ESA 1997) experiment and more accurate evolutionary track computations for pre- and post-main sequence stars became available in the last years; bearing this in mind, we are looking, in the present paper, for refined ages to distinguish physical from optical systems. It is timely to do such work due to new photometric observations and their calibrations, new stellar evolutionary models and Hipparcos parallaxes.

Using as a starting point the parallaxes of the primaries measured by Hipparcos, and by forcing the companion candidates to be at the same distance as the primaries, we derive a new estimation of the ages of each system component and discuss the consistency of these ages.

The evolutionary status of each system will be analysed through the position of the components in the HR diagram among evolutionary tracks. This requires the knowledge of $T\rm _{eff}$, $\log g$ and the absolute magnitude.

In Sect. 3 we discuss the parameters of the early type stars and the extinction determination. In Sect. 4 the parameters of the late type stars are determined. In Sect. 5 the position of stars in the HR diagram is presented from which the age of the stars is derived. Uncertainties in the observed parameters and systematic errors in the various calibrations used are taken into account to derive these ages. Differences in the ages obtained by using various sources of evolutionary models are considered. The reliability of the physical nature of each system is analysed in Sects. 6 and 7 we discuss the rôle of the age determination as a criterion to select physical pairs among these systems.