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