Exact Multiplets of Spontaneously Broken Discrete Global Symmetries: the   Example of $N=2$ Susy QCD
  In these notes, we emphasize the r\^ole of spontaneous broken global discrete symmetries acting on the moduli space of $N=2$ susy Yang-Mills theories and show how they can be used, together with the BPS condition, as a spectrum generating symmetry. In particular, in the strong-coupling region, all BPS states come in multiplets of this broken symmetry. This played a key r\^ole in the determination of the strong-coupling spectra. 
Adel Bilal and Frank Ferrari (ENS Paris)