We study strong coupling regimes in compactifications of the heterotic string to 4d with N=1 supersymmetry. We show that perturbative threshold corrections to the heterotic string lead to strong coupling singularities that do not have a counterpart in the analogue N=2 compactifications of Type IIA on Calabi-Yau threefolds. These strong coupling singularities naively cut off the moduli space leading to a boundary of the moduli space at finite distance. However, non-perturbative effects are expected to partially resolve these singularities allowing to continue the moduli space past them. Using duality to M-theory we are able to compute these non-perturbative corrections and show how these resolve strong coupling singularities.
This talk is based on work to appear with Mirjam Cvetic.