Isospin violation and the magnetic moment of the muon

We calculate the leading isospin-violating and electromagnetic corrections for the decay τ−→π0π−ντ at low energies. The corrections are small but relevant for the inclusion of τ decay data in the determination of hadronic vacuum polarization especially for the anomalous magnetic moment of the muon. We show that part of the systematic differences between the measured form factors in τ−→π0π−ντ and e+e−→π+π− is due to isospin violation.