  We calculate the decay of $\tau^-\to\pi^-\gamma\nu_\tau$ in the framework of
Resonance Chiral Theory ($R\chi T$). By demanding the high energy constraints
from QCD on the related form factors, we could predict the various physical
observables of $\tau^-\to\pi^-\gamma\nu_\tau$ without any free parameter. Our
results show that for a realistic cut on the photon energy (around $100$ MeV)
this mode gives a branching ratio of roughly $0.1%$ that should have already
been detected at the heavy-flavour factories. Another interesting subject we
have studied based on our calculation of the decay
$\tau^-\to\pi^-\gamma\nu_\tau$ is the experimental background estimation of the
lepton flavour violation process $\tau^- \to \mu^- \gamma$. We point out that
although the description of radiation that PHOTOS provides -which has been used
by BaBar and Belle collaborations to estimate this source of background- is in
excellent agreement with the theoretical expectations in the low energy region
in $\tau^-\to\pi^-\gamma\nu_\tau$ decay, this is not the case in the high
energy region, precisely where it is easier that this decay mimics the process
$\tau^-\to\mu^-\gamma$.
