On Type IIB Vacua With Varying Coupling Constant
  We describe type IIB compactifications with varying coupling constant in d=6,7,8,9 dimensions, where part of the ten-dimensional SL(2,Z) symmetry is broken by a background with Gamma_0(n) or Gamma(n) monodromy for n=2,3,4. This extends the known class of F-theory vacua to theories which are dual to heterotic compactifications with reduced rank. On compactifying on a further torus, we obtain a description of the heterotic moduli space of G bundles over elliptically fibered manifolds without vector structure in terms of complex geometries. 
P. Berglund, A. Klemm, P. Mayr and S. Theisen