Threshold Corrections in K3× T2 Heterotic String Compactifications

Abstract We consider compactifications of the heterotic string on K3×T2 so that the resulting theory in d = 4 space-time dimensions has N = 2 supersymmetry. The gravitational and gauge coupling constants of the low-energy effective theory receive threshold corrections from loops of super-heavy string states. We calculate these corrections for the case when the K3-surface is a Zn orbifold of a four torus T4. The results are used to determine the one-loop prepotential F (1) 0 for the vector multiplets and the gravitational coupling F (1) 1 .