5 Summary

Based on repeated measurements of the relative astrometry in 34 close T Tauri binary systems, we have reproduced the results given by G95 and extended their work to a larger number of binaries and particularly to a much longer timespan of up to 10 years. We showed that in most systems significant relative motion of the components has occured. In almost all cases this relative motion can be explained by orbital motion. In only two systems the observed motion may be the result of the proper motion of a T Tauri star that is accidentally projected in the close vicinity of a background star.

From the short pieces of orbit available at the moment (up to $20^{\circ}$ in position angle), we derive a mean dynamical system mass of $2.0\pm 0.7~M_{\odot}$ for our sample. This mass is consistent with the predictions of current sets of PMS evolutionary models within the uncertainties. The large formal error of this mean mass does not allow a significant discrimination between different models, but we draw the tentative conclusion that the masses predicted by the D'Antona & Mazzitelli (1998) model may be systematically too low.

The result that orbital motion can be detected in most systems discussed here indicates that the "companions'' found in previous multiplicity surveys really are gravitationally bound stars. This is a further confirmation of the binary overabundance in Taurus-Auriga and Scorpius-Centaurus compared to nearby main sequence stars. Furthermore, the detection of orbital motion allows a definite classification of three objects with very close separations as stellar companions.

Acknowledgements

We thank the staff at ESO La Silla and Calar Alto for their support during several observing runs. In particular we are grateful to Andreas Eckart and Klaus Bickert for their support in observing with the SHARP I camera. We modified a program written by Sabine Frink to carry out the computer simulations described in Sect. 3.3. The authors appreciate fruitful discussions with Michal Simon, and thank the anonymous referee for fair and constructive criticism.