  Contrary to some previous claims, we find a sizable instanton contribution to
the finite energy sum rule used to extract the value of the strong coupling
from the measured $\tau$ decay widths. It is of the same order of magnitude as
standard nonperturbative corrections induced by vacuum quark and gluon
condensates. Our result indicates that there might be no hierarchy of power
corrections in finite energy sum rules at the scale of $\tau$ mass. Therefore,
the standard nonperturbative corrections do not necessarily improve the
accuracy of the theoretical predicition, but can rather be used to estimate an
intrinsic accuracy of the pure perturbative calculation, which turns out to be
rather high on this evidence, of order one percent.

High Energy Physics - Phenomenology