On type IIB vacua with varying coupling constant

Abstract 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 γ 1 ( n )orγ( 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.