Threshold effects in open-string theory
  We analyze the one-loop effective gauge-field action in $Z_2$-orbifold compactifications of type-I theory. We show how, for non-abelian group factors, the threshold effects are ultraviolet finite though given entirely by a six-dimensional field theory expression. 
C. Bachas and C. Fabre ( CPTH, Ecole Polytechnique )