8 Summary

From speckle interferometry and direct imaging we have derived resolved JHK photometry for the individual components of T Tauri binary systems in nearby star forming regions. These measurements are combined with other data taken from literature (resolved JHK photometry from other authors, system magnitudes and spectral types, extinction coefficients) to study properties of the components in young binary systems. The main results are:

• We have found only very few unusually red objects that may be young substellar objects or infrared companions. Their number is too small to have significantly influenced the binary statistics in Taurus-Auriga.
• The placement of the components into NIR color magnitude diagrams is affected by large errors and thus allows no precise determination of stellar ages from PMS evolutionary models. We can however detect problematic cases and find that V 819 Tau B is probably an unrelated background object. The locations of the components of the 16 other WTTS systems into the the CMDs are in line with the assumption that all components within a system are coeval.
• The determination of masses from the HRD has been performed using the following procedure: we derive stellar luminosities from the J-band magnitudes, assign the optical system spectral type to the primary and use the assumption that all components within one system are coeval.
• The use of three different sets of PMS tracks then yields mass functions that are different at the 99% confidence level. For this reason we discuss the results of all three models used separately. These differences tend to be larger than the uncertainties resulting from observational errors. In addition, the latter mass errors are random and thus partially cancel in a statistical discussion.
• Within the uncertainties the distribution of mass ratios is flat for $M_2/M_1\ge 0.2$. There are no significant correlations between mass ratio and projected separation or mass ratio and primary mass. These results are in line with the wideley accepted idea that binaries are formed by fragmentation during protostellar collapse processes. Moreover, they suggest that the final masses of the components are largely determined by fragmentation itself and not by subsequent accretion.

Acknowledgements

We are grateful to Andreas Eckart and Klaus Bickert for their support in observing with the SHARP camera. We also thank the staff at ESO La Silla and Calar Alto for their support during several observing runs. The authors appreciate fruitful discussions with Michael Meyer, Monika Petr, Matthew Bate and Coryn Bailer-Jones, and they thank an anonymous referee for pointed and productive comments. This research has made use of the SIMBAD database, operated at CDS, France.