Superconformal Partition Functions and Non-perturbative Topological   Strings
  We propose a non-perturbative definition for refined topological strings. This can be used to compute the partition function of superconformal theories in 5 dimensions on squashed S^5 and the superconformal index of a large number of 6 dimensional (2,0) and (1,0) theories, including that of N coincident M5 branes. The result can be expressed as an integral over the product of three combinations of topological string amplitudes. SL(3,Z) modular transformations acting by inverting the coupling constants of the refined topological string play a key role. 
Guglielmo Lockhart and Cumrun Vafa
hep-th