A note on isospin violation in Pl2(gamma) decays
  We discuss the size of isospin violating effects of both electromagnetic (e \neq 0) and strong (m_u \neq m_d) origin in P_{\ell 2 (\gamma)} decays. Our results are relevant for the extraction of |V_{us}/V_{ud}| from the measured ratio of the K_{\mu 2 (\gamma)} and \pi_{\mu 2 (\gamma)} decay widths combined with F_K/F_\pi obtained in lattice calculations in the isospin limit. We point out that strong isospin-breaking corrections, neglected in previous studies, enter at the same level as the electromagnetic effects. The updated value for the ratio of CKM elements reads |V_{us}/V_{ud}| = 0.2316(12). 
Vincenzo Cirigliano and Helmut Neufeld
hep-ph