Résumé
Twistor space provides an efficient description of quaternion-Kahler (QK) manifolds. In physics, the latter appear as supergravity moduli spaces and often carry an isometric action of the S-duality group SL(2,Z). I’ll show that a lift of S-duality to the twistor space gives rise to a rich and interesting structure, and describe its consequences for a class of QK manifolds arising in Calabi-Yau string compactifications. In particular, this includes mock modularity of generating functions of DT invariants and the weak/strong coupling duality of topological string partition function.