  We 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.
