defined in Eq.(\[dg2\]). As an explicit example, if we take the mass of the muon to be zero then our differential decay rate for $D\rightarrow   K^{*}
\mu\nu_{\mu}$ at $q^{2}=0$ gives a value of $5.8$ in units of $|V_{cs}|^{2}10^{10}sec^{-1}GeV^{-2}$. The threshold for non-zero mass is $q^{2}=0.011  \; GeV^{2}$ where the decay rate vanishes. It is at $q^{2}=0.087  \; GeV^{2}$ that the value of $5.8$ is first obtained. However, this $5.8$ is now composed of three parts (see table \[tab2\]), that coming from our $A_{0}(q^2)$ with a value of $4.15$ and two other parts, one called a flip contribution (giving $1.04$) with the remaining contribution of $0.62$ coming from the transverse helicity part. In our model the ratio $[d\Gamma (D\rightarrow \rho \mu \nu_{\mu}
)/dq^{2}]    /[d\Gamma    (D\rightarrow   K^{*}   \mu   \nu_{\mu}
)/dq^{2}]_{m_{\mu}\neq 0}$ at this point ($0.087$) becomes $0.96$ compared to $0.97$ at $q^{2}=0$ in the massless limit. So, the $SU(3)_{F}$ limit remains steady when the simple, single form-factor is replaced by a more complicated collection of form factors. This behaviour breaks down only at higher $q^2$ when the effects of the differing masses of $\rho$ and $K^*$ become obvious. Figures \[Fig2\] and \[Fig3\] show the effect of a massive muon in $D\rightarrow \rho 