On the strong coupling behavior of heterotic and type I orbifolds
  We study the bulk effective theory of a class of orbifolds of the type IIB string with D5-branes, compactified to four dimensions. These constructions are connected, in a region of their moduli space, to some orbifolds of the type I and heterotic string. We compare the effective actions through the coupling of the R^2 term, and we argue that these orbifolds provide non-perturbative deformations of the latter, in which the gauge group is entirely non-perturbative. 
Andrea Gregori