1 Introduction

The discovery by the Einstein X-ray satellite that many low-mass pre-main sequence (PMS) stars are strong X-ray sources (Walter 1986), led many researchers to pursue the search for X-ray emitting low-mass pre-main sequence stars. A lot of progress in this field has been accomplished in the last decade by the observations of the ROSAT satellite which detected many new weak T Tauri stars (WTTS), not only in the star forming clouds, but also in the surroundings of star formation complexes (see Feigelson & Montmerle 2000; Walter et al. 2000, and references therein).

The origin of the scattered population of WTTS has been somewhat debated. One of the main reasons for such a debate has been the low spectroscopic resolution used for the identification of the WTTS candidates, because the presence of strong Lithium $\lambda$6707 absorption cannot be unambiguously assessed. Some researchers argued that such objects are not PMS stars, but young zero-age main-sequence (ZAMS) stars (Briceño et al. 1997; Favata et al. 1997; Micela et al. 1993). However, several investigations, based on high-resolution spectroscopy, demonstrated that most of the widely scattered WTTS are indeed low-mass PMS stars (Covino et al. 1997; Wichmann et al. 1999; Alcalá et al. 2000). Feigelson (1996) suggested that the large number of scattered WTTS were formed in cloudlets which dissipated immediately after star formation. Other authors proposed that a considerable number of these stars may be members of the Gould Belt (Wichmann et al. 1997; Guillout et al. 1998).

The follow-up observations of the ROSAT discovered WTTS using high-resolution spectroscopy have also allowed the identification of a considerable number of PMS spectroscopic binaries among these stars. For instance, several double line spectroscopic binaries were discovered in the Chamaeleon (Covino et al. 1997), Lupus (Wichmann et al. 1999) and Orion (Alcalá et al. 2000) star forming regions (SFRs), which have increased significantly the number statistics of PMS spectroscopic binaries Melo et al. (2001). The PMS binaries are of crucial importance because the determination of dynamical masses, by the solution of their orbits, allows to put constraints on the theoretical PMS evolutionary tracks. Recently, Covino et al. (2001) have solved the orbits of some of these systems in Orion, and the first eclipsing PMS binary with solar-mass components was also discovered among that sample (Covino et al. 2000). In a ROSAT pointed observation on the region of the B-type star $\beta$ Cru, Park & Finley (1996, hereafter PF96) found a group of six X-ray emitting stars to be good candidates for WTTS. They suggested that these stars might be members of a previously unrecognized star forming region which includes $\beta$ Crux itself. Feigelson & Lawson (1997, henceforth FL97) used low-resolution spectroscopy to sudy these objects and concluded that they are PMS stars, although not forming part of a previously unrecognized star forming region, but being members of the low-mass PMS population of the Sco-Cen association, and representing just a few of the many WTTS in Sco-Cen.

In this paper, we characterize the Crux stars of the PF96 sample by means of high-resolution spectroscopic observations and near infrared imaging. In Sect. 2, we present the observations and data reduction. In Sect. 3, we describe the determination of radial and rotational velocities, as well as of the effective temperatures and line equivalent widths. In Sect. 4, we report the discovery of two binaries in the sample, namely a close visual binary, and a double-lined spectroscopic binary (SB2), whose orbital solution is also presented. Finally, we discuss the PMS nature of the stars in Sect. 5.