ON LOOP CORRECTIONS TO STRING EFFECTIVE FIELD-THEORIES - FIELD-DEPENDENT GAUGE COUPLINGS AND SIGMA-MODEL ANOMALIES

We show that certain one-loop corrections to superstring effective four-dimensional lagrangians, involving non-harmonic field-dependent renormalization of gauge couplings, can be consistently written in a standard N = 1 supergravity form, preserving target-space duality. The preservation of target-space duality is due both to a four-dimensional Green - Schwarz mechanism and to local terms, coming from non-local chiral superfields, originated by mixed gauge-sigma-model anomaly diagrams. In some models, the Green-Schwarz mechanism is sufficient to achieve complete anomaly cancellation. In more general models automorphic functions, generated by the integration over the heavy string modes, are required to preserve target-space duality.