One-loop Effective Action in Open-string theory

This is a summary of the analysis of the one-loop effective action in Z_2-orbifold compactifications of type-I theory presented in references [1,2]. We show how, for non-abelian group factors, the threshold effects are ultraviolet finite though given entirely by a six-dimensional field theory expression. We also discuss implications for the equivalence between Type-I and Heterotic four dimensional N=2 superstring theories.