2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55257The paper containes a classification of consistent free string models in physical dimensions and a brief discussion of recent results concerning relations between various models.9 pages, LaTeX,Talk given at NATO Advanced Research Workshop on Theoretical Physics: New Developments in Quantum Field Theory, Zakopane, Poland, 14-20 Jun 1997High Energy Physics - Theorytext