  Within a four-dimensional, toroidally compactified heterotic string, we
identify (quantized) charge vectors of electrically charged BPS-saturated
states (along with the tower of SL(2,Z) related dyonic states), which preserve
1/2 of N=4 supersymmetry and become massless along the hyper-surfaces of
enhanced gauge symmetry of the two-torus moduli sub-space. In addition, we
identify charge vectors of the dyonic BPS-saturated states (along with the
tower of SL(2,Z) related states), which preserve 1/4 of N=4 supersymmetry, and
become massless at two points with the maximal gauge symmetry enhancement.
