Companions usually have no infrared excess, nor do primaries with massive companions. Furthermore, X-ray emission in some HAeBe stars may well be explained by the presence of a T Tauri companion. First, any stellar formation theory must explain the large abundance of multiple systems among Main Sequence and young low-mass T Tauri stars. Second, binary studies allow the direct determination of physical parameters. HAeBe binaries observed with PUEO at 2.2 and view in 3D projection.
Smallest peak is the infrared companion.
Circles represent the primary star, triangles represent the companion. Dotted line are best fitted stellar models.