Charge fractionization in N=2 supersymmetric QCD

It is shown that the physical ``quark number'' charges which appear in the central charge of the supersymmetry algebra of $N=2$ supersymmetric QCD can take irrational values and depend non trivially on the Higgs expectation value. This gives a physical interpretation of the constant shifts which the ``electric'' and ``magnetic'' variables $a_D$ and $a$ undergo when encircling a singularity, and show that duality in this model is truly an electric-magnetic-quark number duality. Also included is a computation of the monodromy matrices directly in the microscopic theory.