  We study compactifications of type IIB supergravity on Calabi-Yau threefolds.
The resulting low energy effective Lagrangian is displayed in the large volume
limit and its symmetry properties - with specific emphasis on the SL(2,Z) - are
discussed. The explicit map to type IIA string theory compactified on a mirror
Calabi-Yau is derived. We argue that strong coupling effects on the worldsheet
break the SL(2,Z).
