A D=4 N=1 orbifold of type I strings

We consider the propagation of type I open superstrings on orbifolds with 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 consistency 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 intersect. An amusing attribute of these models is the importance of the tree-level (in type I language) superpotential to the consistent relationship between Higgsing and the motions of 5-branes.