Quark Number Fractionalization in N=2 Supersymmetric $SU(2) \times   U(1)^{N_f}$ Gauge Theories
  Physical quark-number charges of dyons are determined, via a formula which generalizes that of Witten for the electric charge, in N=2 supersymmetric theories with $SU(2) \times U(1)^{N_f} $ gauge group. The quark numbers of the massless monopole at a nondegenerate singularity of QMS turn out to vanish in all cases. A puzzle related to CP invariant cases is solved. Generalization of our results to $SU(N_c)\times U(1)^{N_f}$ gauge theories is straightforward. 
Giuseppe Carlino, Kenichi Konishi and Haruhiko Terao