2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/50029We investigate the possibility of lowering the string scale in four dimensional heterotic models possessing a non-perturbative extension of the gauge group. In particular, we consider a class of compactifications in which the perturbative gauge sector is massive, and all the gauge bosons are non-perturbative, with a coupling independent on the Planck and string scales.Latex, 7 pages, version revised in the light of new resultsHigh Energy Physics - TheoryHigh Energy Physics - Phenomenologytext