Isospin violation and the magnetic moment of the muon

We calculate the leading isospin-violating and electromagnetic corrections for the decay tau (-) --> pi (0)pi (-)nu (tau) at low energies. The correct ions are small but relevant for the inclusion of tau decay data in the dete rmination of hadronic vacuum polarization especially for the anomalous magn etic moment of the muon. We show that part of the systematic differences be tween the measured form factors in tau (-) --> pi (0)pi (-)nu (tau) and e()e(-) --> pi (+)pi (-) is due to isospin violation. (C) 2001 Elsevier Scien ce B.V. All rights reserved.