Massless BPS-Saturated States on the Two-Torus Moduli Sub-Space of Heterotic String

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. On sabbatic leave from the University of Pennsylvania.