  Being a determination at low energies, the analysis of hadronic tau decay
data provides a rather precise determination of the strong coupling alpha_s
after evolving the result to M_Z. At such a level of precision, even small
non-perturbative effects become relevant for the central value and error. While
those effects had been taken into account in the framework of the operator
product expansion, contributions going beyond it, so-called duality violations,
have previously been neglected. The following investigation fills this gap
through a finite-energy sum rule analysis of tau decay spectra from the OPAL
experiment, including duality violations and performing a consistent fit of all
appearing QCD parameters. The resulting values for alpha_s(M_tau) are 0.307(19)
in fixed-order perturbation theory and 0.322(26) in contour-improved
perturbation theory, which translates to the n_f=5 values 0.1169(25) and
0.1187(32) at M_Z, respectively.
