THRESHOLD CORRECTIONS AND MODULAR INVARIANCE IN FREE FERMIONIC STRINGS

We examine the modular invariant properties of the low-energy effective action of free fermionic models. This entails a detailed comparison between the so-called string and supergravity bases, since only in the latter are the modular invariant properties evident, whereas the S-matrix calculations are performed in the former. We then adjoin these results to the Kahler anomaly method for obtaining the untwisted moduli dependence of the string threshold corrections to the gauge couplings. Finally we apply our results to the flipped SU (5) string model and determine the common unification scale for all the gauge couplings in the model.