  We discuss perturbative four-dimensional compactifications of both the SO(32)
heterotic and the Type I string on smooth Calabi-Yau manifolds endowed with
general non-abelian and abelian bundles. We analyse the generalized
Green-Schwarz mechanism for multiple anomalous U(1) factors and derive the
generically non-universal one-loop threshold corrections to the gauge kinetic
function as well as the one-loop corrected Fayet-Iliopoulos terms. The latter
can be interpreted as a stringy one-loop correction to the
Donaldson-Uhlenbeck-Yau condition. Applying S-duality, for the Type I string we
obtain the perturbative Pi-stability condition for non-abelian bundles on
curved spaces. Some simple examples are given, and we qualitatively discuss
some generic phenomenological aspects of this kind of string vacua. In
particular, we point out that in principle an intermediate string scale
scenario with TeV scale large extra dimensions might be possible for the
heterotic string.
