Unified models at intermediate energy scales and Kaluza–Klein excitations

We discuss the possibility of intermediate gauge coupling unification in unified models of string origin. Useful relations of the β-function coefficients are derived, which ensure unification of couplings when Kaluza–Klein excitations are included above the compactification scale. We apply this procedure to two models with SU(3)×SU(3)L×SU(3)R and SU(4)×O(4) gauge symmetries.