Centre de Physique Théorique, Ecole Polytechnique, F-91128, Palaiseau, France
b Laboratoire de Physique Théorique, ENS, F-75231, Paris, France
LPT, Bât. 210, Univ. Paris-Sud, F-91405, Orsay, France
Abstract:
We compute threshold effects to gauge couplings in four-dimensional N orientifold models of type I strings with GN = 2 and GN = 1 supersymmetry, and study their dependence on the geometric moduli. We also compute the tree-level (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.