  The tau -> mu nu nu branching ratio has been measured using data collected
from 1990 to 1995 by the OPAL detector at the LEP collider. The resulting value
of B(tau->mu nu nu) = 0.1734 +/- 0.0009(stat) +/- 0.0005(syst) has been used in
conjunction with other OPAL measurements to test lepton universality, yielding
the coupling constant ratios g(mu)/g(e) = 1.0005 +/- 0.0043 and g(tau)/g(e) =
1.0031 +/- 0.0047, in good agreement with the Standard Model prediction of
unity, and also to determine a value for the Michel parameter eta = 0.004 +/-
0.036. This is subsequently used to find a model-dependent limit of the mass
for the charged Higgs boson, m(Higgs) > 1.30 tan(beta), in the Minimal
Supersymmetric Standard Model.
