Appears in collection : 2008 - 50th anniversary of the IHES: Theoretical Physics Conference

1) The low energy expansion of string theory. Higher derivative gravity.

2) Interplay of duality and maximal supersymmetry. Poisson equations on moduli space. Exact non-perturbative coefficients.

3) Duality with eleven-dimensional supergravity compactified on two-torus. Multi-loop Feynman diagrams and modular invariance.

4) Connections with perturbative N=8 supergravity. UV divergence properties.