field strength (magnetic or electric) background that is slowly varying (if not constant), to derive a low energy effective action in an $\alpha'$ expansion. Much important work has been done by using this technique. However for the most part this work has been used only to get an effective field theory valid below the string scale. In particular it does not make sense to consider the behavior of a term like $\frac{1}{g^{2}(\mu^{2})}{\rm tr}(F_{u\nu}F^{u\nu})$ when the momentum flowing through this operator is greater than the string scale. The representation in terms of such a local operator simply breaks down at these energies. In particular this means that any argument which purports to have a low (around say few TeV) string scale and gauge coupling unification at some much higher scale (such as the standard GUT scale of $10^{16}GeV$), cannot possibly make sense.

In this work we will address the question of string threshold corrections to gauge couplings by first reviewing the literature. The issue came up with Kaplunovsky’s calculation of these effects in the context of the heterotic string [@Kaplunovsky:1987rp]. The question that arose was how to account for moduli dependent corrections that could not be written as harmonic functions of 