Isospin symmetry breaking and gluon anomaly

The contribution of the electromagnetic interaction to the self-energy of pseudo-Goldstone bosons, as well as to the mixing angles and weak decay constants, is calculated in the first nonleading order in the simultaneous expansion in powers of [Formula: see text], momenta and quark masses. Particular attention is paid to the isospin symmetry breaking which is generated by means of [Formula: see text] and [Formula: see text] admixtures to the pion. It is shown that the gluon anomaly enhances the electromagnetic contribution to the [Formula: see text]-[Formula: see text] and [Formula: see text]-[Formula: see text] mixing angles, which facilitates the restoration of isospin symmetry in the spectrum of pseudo-Goldstone states at next-to-leading order. The contribution of the electromagnetic interaction to the [Formula: see text]-[Formula: see text] mixing angle is shown to be only about 3%.