  We review the argument for field redefinitions arising from threshold
corrections to heterotic string gauge couplings, and the relation between the
linear and the chiral multiplet. In the type IIB case we argue that the
necessity for moduli mixing at one loop order has not been clearly established,
since this is based on extending the background field expansion way beyond its
regime of validity. We also resolve some issues related to the form of
non-perturbative terms resulting from gaugino condensation. This enables us to
estimate the effective cutoff in the field theory by evaluating the
non-perturbative superpotential by two different methods, and find that it is
around the Kaluza-Klein scale, as one might have expected on general grounds of
self-consistency.
