1 Introduction

The most fundamental parameter of a star is its mass, which determines almost everything about its birth, life, and death. For low mass pre-main sequence stars this parameter is practically always derived by comparing the location of the star in the Hertzsprung-Russell (HR) diagram with theoretically calculated evolutionary tracks, i.e. it has not been determined directly. Since the evolutionary tracks published by different groups differ a lot, it is necessary to test them thoroughly. A good way to do this is by comparing the masses derived from the evolutionary tracks with dynamically determined masses. Direct determinations of the mass are possible if the object is a double-lined spectroscopic binary, and if additionally the inclination is known.

The inclination can be determined either if the system is eclipsing, or if it is spatially resolved, or if the motion of the photo-centre is measured astrometrically. In some special cases the inclination might also be estimated indirectly. For example, in the case of a circumbinary disk, it is reasonable to assume that the inclination of the orbit is roughly the same as the disk. In the near future the determination of the masses of many pre-main sequence stars will be possible when the VLTI becomes available. Additionally, upcoming space astrometry missions (FAME, DIVA, SIM, GAIA) will allow us to measure the motion of the photo-centre for many spectroscopic binaries. In order to prepare for these projects, we have thus initiated a survey for pre-main sequence spectroscopic binaries located in nearby star-forming regions. Additionally, we also derive the orbits of recently discovered double-lined spectroscopic binaries.

However, even if the inclination of the system is not known, in double-lined spectroscopic binary systems the mass ratios of the components can still be used to constrain evolutionary tracks. Thus, these objects are also of interest. We present here the orbital solution of the double-lined spectroscopic binary RX J1603.8-3938. Although RX J1603.8-3938 is a short period binary, and it is neither eclipsing nor a suitable target for the VLTI, it turns out to be quite interesting for our understanding of early stellar evolution.

Apart from being useful for deriving the masses, the pre-main sequence binaries are also interesting as such. First of all, the mass ratios of binaries of different orbital periods may hold important clues for the formation of these systems. Secondly, the evolution of the orbital elements gives important clues on the tidal interaction of the binary. Duquennoy & Mayor (1991) find for stars in the solar neighbourhood that binaries with periods of 11d or less have circular orbits, and binaries with longer periods have a mean eccentricity of $0.31\pm 0.04$. Zahn & Bouchet (1989) studied the tidal interaction theoretically, and found that almost all of the orbital circularisation of low-mass stars occurs during the pre-main sequence phase of evolution. Mathieu (1992) in fact found that the transition period between circular and eccentric orbits is apparently between 4 and 5 d for pre-main sequence stars, and thus significantly shorter than for main-sequence stars. However, the number of pre-main sequence binary systems with solved orbits is still quite low, and surprises can be expected when more orbits are solved. By studying pre-main sequence binaries we can thus learn more about tidal interaction, especially on the timescales on which this process works. Since pre-main sequence stars are quite frequently covered with spots, they are often suitable for investigating whether short-period systems have their rotation synchronised with the orbital motion. The only pre-main sequence spectroscopic binary where the synchronization of the rotational periods of the components have been studied up to now is V773Tau (Rydgren & Vrba 1993; Welty 1995).

For this system it was found that the rotational period of the stars is about three days, whereas the orbital period of the system is about 51 days. Last but not least, short-period pre-main sequence stars often show significant amounts of stellar activity. They are often strong X-ray sources, and very large flares have been reported. Non-thermal radio emission has been detected in some cases. Thus, short period pre-main sequence binaries are very suitable, in order to learn more about the generation of magnetic fields, and the stellar activity in pre-main sequence stars. The best studied case in this respect is again V773 Tau (Guenther et al. 2000).