We work out the relation between automorphic forms on SO(2 + s, 2, Z) and gauge one–loop corrections of heterotic K3×T 2 string compactifications for the cases s = 0, 1. We find that one–loop gauge corrections of any orbifold limit of K3 can always be expressed by their instanton numbers and generic automorphic forms. These functions classify also one– loop gauge thresholds of N=1 (0, 2) hetero...