PERTURBATIVE COUPLINGS OF VECTOR MULTIPLETS IN N=2 HETEROTIC STRING VACUA

We study the low-energy effective Lagrangian of N = 2 heterotic string vacua at the classical and quantum level. The couplings of the vector multiplets are uniquely determined at the tree level, while the loop corrections are severely constrained by the exact discrete symmetries of the string vacuum. We evaluate the general transformation law of th e perturbative prepotential and determine its form for the toroidal co mpactifications of six-dimensional N = 1 supersymmetric vacua.