  We study relevant deformations of the N=2 superconformal theory on the
world-volume of N D3 branes at an A_{k-1} singularity. In particular, we
determine the vacuum structure of the mass-deformed theory with N=1
supersymmetry and show how the different vacua are permuted by an extended
duality symmetry. We then obtain exact, modular covariant formulae (for all k,
N and arbitrary gauge couplings) for the holomorphic observables in the massive
vacua in two different ways: by lifting to M-theory, and by compactification to
three dimensions and subsequent use of mirror symmetry. In the latter case, we
find an exact superpotential for the model which coincides with a certain
combination of the quadratic Hamiltonians of the spin generalization of the
elliptic Calogero-Moser integrable system.
