Isospin Symmetry Breaking and the $\rho-\omega$-System

Simple quark models for the low lying vector mesons suggest a mixing between the u- and d-flavors and a violation of the isospin symmetry for the $\rho-\omega$- system much stronger than observed. It is shown that the chiral dynamics, especially the QCD anomaly, is responsible for a restoration of the isospin symmetry in the $\rho-\omega$-system.