  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.
