A D=4 N=1 ORBIFOLD OF TYPE-I STRINGS

We consider the propagation of type I open superstrings on orbifolds w ith four non-compact dimensions and N = 1 supersymmetry. In this paper , we concentrate on a non-trivial Z(2) x Z(2) example. We show that co nsistency conditions, arising from tadpole cancellation and algebraic sources, require the existence of three sets of Dirichlet 5-branes. We discuss fully the enhancements of the spectrum when these 5-branes in tersect. An amusing attribute of these models is the importance of the tree-level (in type I language) superpotential to the consistent rela tionship between Higgsing and the motions of 5-branes.