Gauge couplings in four-dimensional Type I string orbifolds

We compute threshold effects to gauge couplings in four-dimensional Z(N) or ientifold models of type I strings with N = 2 and N = 1 supersymmetry, and study their dependence on the geometric moduli. We also compute the tree-le vel (disk) couplings of the open sector gauge fields to the twisted closed string moduli of the orbifold in various models and-study their effects and that of the one-loop threshold corrections on gauge coupling unification. We interpret the results from the (supergravity) effective theory point of view and comment on the conjectured heterotic-type I duality. (C) 1999 Else vier Science B.V. All rights reserved.