On interpolations from SUSY to non-SUSY strings and their properties

The interpolation from supersymmetric to non-supersymmetric heterotic theories is studied, via the Scherk-Schwarz compactification of supersymmetric theories to . A general modular-invariant Scherk-Schwarz deformation is deduced from the properties of the theories at the endpoints, which significantly extends previously known examples. This wider class of non-supersymmetric theories opens up new possibilities for model building. The full one-loop cosmological constant of such theories is studied as a function of compactification radius for a number of cases, and the following interpolating configurations are found: two supersymmetric theories related by a -duality transformation, with intermediate maximum or minimum at the string scale; a non-supersymmetric theory interpolating to a supersymmetric theory, with the theory possibly having an AdS minimum; a ``metastable'' non-supersymmetric theory interpolating via a theory to a supersymmetric theory.