On four-dimensional N=1 Type I compactifications

We consider four-dimensional N = 1 supersymmetric Type I compactifications on toroidal orbifolds T-6/Gamma. In particular, we focus on the Type I vacu a which are perturbative from the orientifold viewpoint, that is, on the co mpactifications; with well-defined world-sheet expansion. The number of suc h models is rather constrained. This allows us to study all such vacua. Thi s, in particular, involves considering compactifications with non-trivial N S-NS antisymmetric tensor backgrounds. We derive massless spectra for these models, and also compute superpotentials. We review the reasons responsibl e for such a limited number of perturbative Type I compactifications on tor oidal orbifolds (which include Abelian as well as non-Abelian cases). As an aside, we generalize the recent work on large N gauge theories from orient ifolds to include a non-Abelian orbifold. This also provides an important i ndependent check for perturbative consistency of the corresponding Type I c ompactification. (C) 1998 Elsevier Science B.V.