UNIVERSALITY PROPERTIES OF N=2 AND N=1 HETEROTIC THRESHOLD CORRECTIONS

In the framework of heterotic compactifications, we consider the one-l oop corrections to the gauge couplings, which were shown to be free of any infra-red ambiguity. For a class of N = 2 models, namely those th at are obtained by toroidal compactification to four dimensions of gen eric six-dimensional N = 1 ground states, we give an explicit formula for the gauge-group-independent thresholds, and show that these are eq ual within this class, as a consequence of an anomaly cancellation con straint in six dimensions. We further use these results to compute the (N = 2)- sector contributions to the thresholds of N = 1 orbifolds. W e then consider the full contribution of N = 1 sectors to the gauge co uplings, which generically are expected to modify the unification pict ure. We compute such corrections in several models. We finally comment on the effect of such contributions to the issue of string unificatio n.