On the string coupling in a class of stringy orbifold GUTs
  In this short note, we examine the relationship between the string coupling constant, $\gstring{}$, and the grand unified gauge coupling constant, $\alphagut{}$, in a highly successful class of models based on anisotropic orbifold compactifications of the weakly coupled heterotic string. These models represent a stringy embedding of SU(6) gauge-Higgs unification in a five dimensional orbifold GUT. We find that the requirement that the theory be perturbative provides a non-trivial constraint on these models. Interestingly, there is a correlation between the proton decay rate (due to dimension six operators) and the string coupling constant. Finally, we make some comments concerning the extension of these models to the six (and higher) dimensional case. 
Ben Dundee and Stuart Raby