2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/192168We formulate sigma-model duality transformations in terms of spin connection. This allows to investigate the symmetry of the string action including higher order $α'$ corrections. An important feature of the new duality transformations is a simple homogeneous transformation rule of the spin connection (with torsion) and specifically adjusted transformation of the Yang-Mills field. We have found that under certain conditions this duality is a symmetry of the full effective string action in the target space, free of $α'$ corrections. We demonstrate how the exact duality generates new fundamental string solutions from supersymmetric string waves.23 p, Stanford University preprint SU-ITP-93-37 (some references added)High Energy Physics - Theorytext