  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.
