Masses and widths of the rho(770)
  Isospin violation in the $\rho(770)$ mass and width is considered within the $S$ matrix approach using combined fits to the $e^+e^- \to\pi^+\pi^-$ and $\tau^- \to \nu_{\tau}\pi^-\pi^0$ data performed by the ALEPH collaboration. We show that the pole position following from the parameters obtained from the ALEPH fits are not sensitive to the details of the parametrization. In this context, we have found that the pole mass difference and the pole width difference between the charged and neutral $\rho$ are consistent with zero. We show that a one loop calculation including vector, axial vector and pseudo-scalar mesons can satisfactorily describe the observed isospin breaking. We also give an estimate for the mass difference between the neutral and charged states of the $a_1(1260)$. 
M. Feuillat, J.L. Lucio M and J. Pestieau