GRAND UNIFIED STRING MODELS AND LOW-ENERGY COUPLINGS

Recent calculations have shown that the string unification scale is naturally about two orders of magnitude larger than a typical supersymmetric grand unification scale compatible with the latest LEP data on sin2-theta(w) and alpha-3. Studying the evolution of the gauge couplings of the string derived models based on the SU(4) x SU(2)L x SU(2)R and SU(3)3 gauge groups. We examine the conditions that make consistent the high string unification scale with the low energy phenomenology. Assuming only the content of the minimal supersymmetric standard model below a reasonable grand unification scale, we derive the constraints on the string spectrum of the above models.