  We study the disintegration $\tau^- \to \pi^-\ell^+\ell^-\nu_\tau$, where $l$
denotes either an electron or a muon. This process requires knowledge about the
$W^*\gamma^*\pi$ interaction, which is modeled using Resonance Chiral Theory.
Within this framework we study the sensitivity of the observables of the
process, such as the branching ratios and the dilepton mass spectra, to the
model dependent contributions. The information of the hadronization of the
quark currents is encoded in form factors which are evaluated using the
lightest vector and axial-vector multiplets as degrees of freedom. We determine
the couplings by matching the asymptotic QCD behavior of the form factors in
the limit of an infinite number of colours. The branching ratios we obtain show
that the process with $l=e$ should be measured soon in $B$ factories.
