Gauge couplings in four-dimensional Type I string orbifolds

Abstract

We compute threshold effects to gauge couplings in four-dimensional $Z_N$ orientifold models of type I strings with $\mathcal{N}=2$ and $\mathcal{N}=1$ 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

Comments: 54 pages, LaTeX, comments and references added, final version to appear in Nucl. Phys. B