We 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.Comment: 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