The prepotential of N=2 SU(2) x SU(2) supersymmetric Yang-Mills theory with bifundamental matter

We study the non-perturbative, instanton-corrected effective action of the N=2 SU(2) x SU(2) supersymmetric Yang-Mills theory with a massless hypermultiplet in the bifundamental representation. Starting from the appropriate hyperelliptic curve, we determine the periods and the exact holomorphic prepotential in a certain weak coupling expansion. We discuss the dependence of the solution on the parameter q=L2^2/L1^2 and several other interesting properties.