Variations on non-supersymmetric strings

We discuss two different themes in non-supersymmetric strings. First we analyze the backreaction of dilaton tadpoles on the background in a prototype example with 8-branes. Our analysis of the solutions for the warped metric, and the dilaton profile confirms a process of spontaneous compactification in non-supersymmetric strings. Secondly we consider the construction of non-supersymmetric closed strings in various dimensions using orbifolds. We partially classify tachyon-free heterotic models and compute the spectrum of several tachyonic type 11 and heterotic examples. We also discuss the relation to compactification on K3 and Calabi-Yau manifolds.