A note on isospin violation in Pℓ2(γ) decays

We discuss the size of isospin violating effects of both electromagnetic (e≠0) and strong (mu≠md) origin in Pℓ2(γ) decays. Our results are relevant for the extraction of |Vus/Vud| from the measured ratio of the Kμ2(γ) and πμ2(γ) decay widths combined with FK/Fπ 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 |Vus/Vud|=0.2316(12).