Multiplicity of X-ray-selected T Tauri stars in Chamaeleon

We report on a multiplicity survey of a sample of X-ray-selected young stars in the Chamaeleon association. We used speckle interferometry and direct imaging to fond companions in the separation range After correction for chance alignment with background stars, we find a multiplicity 0.13-6. (number of binaries or multiples divided by number of systems) of 14.0% +/- 4.3% and a companion-star frequency (number of companions divided by number of systems) of 14.7% +/- 5.1%. Compared with solar-type main-sequence stars, the companion-star frequency is lower by a factor of 0.61 +/- 0.27. This is remarkably different from the high multiplicity found in the Taurus-Auriga star-forming region and for T Tauri stars in Chamaeleon known before ROSAT. We find only a few binaries with projected separations of more than 70 AU, also in contrast to the results for stars known before ROSAT. This indicates that the X-ray-selected stars belong to a different population than the stars known before ROSAT, a hypothesis further supported by their Hipparcos distances and proper motions.