Electromagnetic masses of the mesons and the generalization of Dashen's theorem

In the framework of $U(3)_L\times U(3)_R$ chiral field theory, Dashen's theorem is reexamined, and the well-known result of $m_{\pi^\pm}^2-m_{\pi^0}^2$ obtained by Das, Guralnik, Mathur, Low, and Young is reproduced. We find that Dashen's theorem, which automatically holds for pseudoscalar mesons in this theory, can be generalized to the sector of axial-vector mesons, however, fails for the sector of vector mesons.