RX J1603.8-3938 -: a surprising pre-main sequence spectroscopic binary

We have determined the orbit of the double-lined spectroscopic binary RX J1603.8-3938. The binary consists of two weak-line T Tauri stars, both of which have a spectral type between K3 and K4. The period of the circular orbit is 7.55626 +/- 0.00021 days and the mass ratio 0.9266 +/- 0.0963. To our knowledge RX J1603.8-3938 is thus the pre-main sequence binary with the longest period that has a circular orbit. Despite the fact that the masses and spectral types of the two components are almost identical, the photospheric lines are much stronger in one component than in the other. In the wavelength region between 5500 and 7800 Angstrom we find that the ratio of the equivalent widths of the primary to the secondary is 0.60 +/- 0.03. This ratio is constant in time: and is the same for all photospheric lines. Since the components are weak-line T Tauri stars, the effect cannot be explained by any kind of veiling. We are led to the conclusion that the secondary is 0.55 +/- 0.05 mag fainter than the primary. It thus turns out that evolutionary tracks of pre-main sequence single stars are unable to explain the position of this system in the HR diagram.