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(1)(n) or Gamma(n) monodromy for n = 2 , 3, 4. This extends the known class of F-theory vacua to theories which ar e dual to heterotic compactifications with reduced rank. On compactifying o n 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. (C) 1999 Published by Elsevier Science B.V. Al l rights reserved.