  The OPERA long-baseline neutrino-oscillation experiment has observed the
direct appearance of $\nu_\tau$ in the CNGS $\nu_\mu$ beam. Two large muon
magnetic spectrometers are used to identify muons produced in the $\tau$
leptonic decay and in $\nu_\mu^{CC}$ interactions by measuring their charge and
momentum. Besides the kinematic analysis of the $\tau$ decays, background
resulting from the decay of charmed particles produced in $\nu_\mu^{CC}$
interactions is reduced by efficiently identifying the muon track. A new method
for the charge sign determination has been applied, via a weighted angular
matching of the straight track-segments reconstructed in the different parts of
the dipole magnets. Results obtained for Monte Carlo and real data are
presented. Comparison with a method where no matching is used shows a
significant reduction of up to 40\% of the fraction of wrongly determined
charges.
