2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/148333We construct a class of superstring solutions in non trivial space-time. The existence of an $N=4$ world-sheet superconformal symmetry stabilizes our solutions under perturbative string loop corrections and implies in target space some unbroken space-time supersymmetries.8 pages, no figures, preprint (Talk given at the International Europhysics Conference on High Energy Physics, Marseille, 22-28 July 1993) CERN-TH.7169/94 (Missprints corrected and title changed)High Energy Physics - Theorytext