  It is shown that isospin-breaking corrections to the pion-deuteron scattering
length can be very large, because of the vanishing of the isospin-symmetric
contribution to this scattering length at leading order in chiral perturbation
theory. We further demonstrate that these corrections can explain the bulk of
the discrepancy between the recent experimental data on pionic hydrogen and
pionic deuterium. We also give the first determination of the electromagnetic
low-energy constant f1.
