Boris Pioline

Title: D-instantons and twistors

Abstract: The hypermultiplet moduli space $M$ in type IIA (resp IIB)  
string compactifications on a CY manifold $X$ is a quaternionic-Kahler  
space of dimension $4(h^{2,1}(X)+1)$ (resp. $4(h^{1,1}(X)+1)$).  
Contrary to the vector multiplet moduli space, it is expected to  
receive instanton corrections from Euclidean D2-branes (resp odd D- 
branes) wrapping special Lagrangian cycles (resp. complex cycles).  
These corrections are conveniently encoded in the complex contact  
structure of the twistor space $Z(M)$. By combining S-duality and  
mirror symmetry, we obtain the general form of such D-instanton  
corrections, which turn out to be expressable as a sum of  
dilogarithms. Relations to the KS wall-crossing formula, BPS black  
hole counting, and possible extensions to include NS5-brane instantons  
will also be discussed.