Bo Reipurth (IfA, University of Hawaii) Title: Dynamical Evolution of Young Binaries: New Results Abstract: I will discuss the results of two new studies of pre-main sequence binaries. The first study is based on an analysis of a large set of Halpha images obtained with HST to study jets in the Orion Nebula. The extraordinary resolution has allowed us to identify numerous binaries with subarcsecond separations. We have selected 781 stars that fulfill criteria for membership in the Orion Nebula Cluster. Among these, we find 78 multiple systems (75 binaries and 3 triples), of which 55 are new discoveries, in the range from 0.1 to 1.5 arcsec. After careful correction for line-of-sight doubles, we perform a statistical study of these binaries and compare the results with the separation distribution function for field stars and for young loose associations, documenting the paucity of young binaries in the ONC on statistically solid grounds. We also analyze the results as a function of distance from the Trapezium, and find strong evidence for stellar interactions in the cluster potential well. The second study is based on infrared H, K, and L'-band images of 189 Class I sources with a median angular resolution of 0.33 arcsec at L'. We have derived the properties of the 89 companions identified (73 of which are new), verify the excess of Class I binary stars over solar-type main-sequence stars found by Duchene et al. (2004), and document an evolution of the separation distribution function as a function of spectral index, a proxy for evolutionary stage. We interpret these new data in terms of significant dynamical evolution among young multiple systems, and demonstrate a strong dependence of protostellar binary properties on the star forming environment.