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 subspace. In addition, we identify charge vectors of the dyonic BPS-saturated stales (along with the tower of SL(2,Z) related starts), which preserve 1/4 of N = 4 supersymmetry, and become massless at two points with the maximal gauge symmetry enhancement.