  In the framework of heterotic compactifications, we consider the one-loop
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 that are obtained
by toroidal compactification to four dimensions of generic six-dimensional N=1
ground states, we give an explicit formula for the gauge-group independent
thresholds, and show that these are equal within this class, as a consequence
of an anomaly-cancellation constraint in six dimensions. We further use these
results to compute the (N=2)-sector contributions to the thresholds of N=1
orbifolds. We then consider the full contribution of N=1 sectors to the gauge
couplings which generically are expected to modify the unification picture. We
compute such corrections in several models. We finally comment on the effect of
such contributions to the issue of string unification.
