On Four Dimensional N=1 Type I Compactifications

We consider four dimensional N=1 supersymmetric Type I compactifications on toroidal orbifolds T^6/G. In particular, we focus on the Type I vacua which are perturbative from the orientifold viewpoint, that is, on the compactifications with well defined world-sheet expansion. The number of such models is rather constrained. This allows us to study all such vacua. This, in particular, involves considering compactifications with non-trivial NS-NS antisymmetric tensor backgrounds. We derive massless spectra for these models, and also compute superpotentials. We review the reasons responsible for such a limited number of perturbative Type I compactifications on toroidal orbifolds (which include Abelian as well as non-Abelian cases). As an aside, we generalized the recent work on large N gauge theories from orientifolds to include a non-Abelian orbifold. This also provides an important independent check for perturbative consistency of the corresponding Type I compactification.