situation is relatively clear. For the electron, the zero mass approximation is good since the threshold is very close to the massless limit. On the other hand, it is obvious that one has to include lepton mass effects when analysing semileptonic decays involving $\tau $–leptons. Since comparisons of the semileptonic decays into $\rho$ and either electrons or muons will be available soon [@Appel], we now discuss the massive muon case.

Two different aspects have to be considered when lepton mass effects are included in an analysis of semileptonic decays [@korner90]. The kinematics of the decay processes change. There is also a change of a dynamical nature: when the lepton acquires a mass there can also be spin–flip contribution.

When the lepton mass is taken to be nonzero, Eq.(\[dg1\]) becomes [@korner90] $$\begin{aligned}
\frac{d\Gamma}{dq^2}|_{m_{\ell}\neq 0} & = & \frac{G^{2}}{(2\pi)^{3}}\, 
|V_{cQ}|^{2} \, \frac{p_{V} \, (q^{2}-m^{2}_{\ell})^{2}}{12 m_{D}^{2}
 q^{2}} \left[ (|H_{+}|^{2}+|H_{-}|^{2}+|H_{0}|^{2}) \right.\nonumber \\
& & \left. \hspace{2cm} +\left( \frac{m^{2}_{\ell}}{2q^{2}} \right)
(|H_{+}|^{2}+|H_{-}|^{2}+|H_{0}|^{2}+3|H_{t}|^{2})\right] \label{dg2}\end{aligned}$$ In addition to spin 1 contributions, there are off–shell spin 0 contributions proportional to $|H_{t}|^{2}$ where $$\begin{aligned}
H_{t} & = & \frac{2m_{D}p_{V}}{\sqrt{q^{2}}} \left[ A_{0}(q^{2})
-\frac{q^{2}}{2m_{V}(m_{D}+m_{V})}A_{2}(q^{2}) \right] \end{aligned}$$

In Eq.(\[dg2\]), spin flip contributions bring in the characteristic flip factor $m^{2}_{\ell}/2q^{2}$ which vanishes in the 