Aspects of type I string phenomenology

We study different phenomenological aspects of compact, D = 4, N = 1 Type I IB orientifolds considered as models for unification of the standard model and gravity, We discuss the structure of the compactification, string and u nification scales depending on the different possible D-brane configuration s. It is emphasized that in the context of Type I models the M-W/M-Planck h ierarchy problem is substantially alleviated and may be generated by geomet rical factors. We obtain the effective low-energy supergravity Lagrangian a nd derive the form of soft SUSY-breaking terms under the assumption of dila ton/moduli dominance. We also discuss the role of anomalous U(1)'s and of t wisted moduli in this class of theories. A novel mechanism based on the rol e of singularities is suggested to achieve consistency with gauge coupling unification in low string scale models. (C) 1999 Elsevier Science B.V. All rights reserved.