F-Theory with Quantized Fluxes
  We present evidence that the CHL string in eight dimensions is dual to F-theory compactified on an elliptic K3 with a $\Gamma_{0}(2)$ monodromy group. The monodromy group $\Gamma_{0}(2)$ allows one to turn on the flux of an antisymmetric two form along the base. The $B_{\mu \nu}$ flux is quantized and therefore the moduli space of the CHL string is disconnected from the moduli space of F-theory/Heterotic strings (as expected). The non-zero $B_{\mu \nu}$ flux obstructs certain deformations restricting the moduli of elliptic K3 to a 10 dimensional moduli space. We also discuss how one can reconstruct the gauge groups from the elliptic fibration structure. 
Michael Bershadsky, Tony Pantev and Vladimir Sadov