Multiplicity of T Tauri stars in Taurus after ROSAT

We surveyed a sample of 75 T Tauri stars in the Taurus star forming region for companions. These stars were discovered with the help of ROSAT. The separation range covered is 0.13′′ to 13′′, where the lower limit is given by the diffraction limit of the telescope and the upper limit by confusion with background stars. Combined with the results of the preceding survey by Leinert et al. (1993), we now have surveyed a sample of 178 young stars in Taurus, 63 classical, 106 weak-line, and 9 unclassified T Tauri stars. Within this sample, we find 68 binaries, 9 triples, and 3 quadruples. After corrections to account for confusion with background stars and for a bias induced through X-ray selection, we count 74 binaries or multiples with a total of 85 companions in 174 systems. This corresponds to a degree of multiplicity (number of binaries or multiples divided by number of systems) of (42.5 ± 4.9)%, or to a duplicity, measured by the number of companions per system, of (48.9±5.3)%, which is higher by a factor of (1.93 ± 0.26) compared to solar-type main-sequence stars.We find no difference in duplicity between classical and weak-line T Tauri stars. There is a difference between close and wide pairs in the sense that close pairs have a flat distribution of flux ratios, while the flux ratios of wide pairs are peaked towards small values.