2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/54359We are discussing the $S$ \& $T$ duality for special class of heterotic string configurations. This class of solutions includes various types of black hole solutions and Taub-NUT geometries. It allows a self-dual point for both dualities which corresponds to massless configurations. As string state this point corresponds to $N_R=1/2$ and $N_L=0$. The string/string duality is shortly discussed.7 pages, Latex, Talk at Trieste workshop on "Strings, gravity.."High Energy Physics - Theorytext