ON GAUGE COUPLINGS IN STRING THEORY

We investigate the field dependence of the gauge couplings of N = 1 st ring vacua from the point of view of the low energy effective quantum field theory. We find that field-theoretical considerations severely c onstrain the form of the string loop corrections; in particular, the d ilaton dependence of the gauge couplings is completely universal at th e one-loop level. The moduli dependence of the string threshold correc tions is also constrained, and we illustrate the power of such constra ints with a detailed discussion of the orbifold vacua and the (2, 2) ( Calabi-Yau) vacua of the heterotic string.