that have been, are not known very precisely yet because of the smallness of the branching ratios associated with them. On the other hand, the theoretical calculations are hard to estimate because of the nonperturbative character of strong interactions. Here, one may resort to a model, but that introduces uncertainties that are inherent to the model itself. To overcome this difficulty, at least to some degree, many authors have studied, instead of the branching ratios of the semileptonic decays of the particular heavy mesons, their ratios at some particular kinematical points, usually at zero recoil.

For example, Sanda and Yamada [@SY] propose a strategy to get $|V_{ub}|$ by relating the differential decay width of the $\bar{B}  \rightarrow  \rho \, \ell \, \bar{\nu}$ to that of the process $\bar{B}  \rightarrow  K^{*} \, \ell \,  \bar{\ell }$ at their respective $q^{2}_{max}$ limits ($(m_{\bar{B}}   -
m_{\rho})^2$ and $(m_{\bar{B}}  -  m_{K^{*}})^2$) using the $SU(3)$–flavor symmetry and heavy quark approximation. They find

$$\frac{|V_{ub}|^{2}}{|V_{tb}V^{*}_{ts}|^{2}}=
\frac{q^{2\bar{B}\rightarrow       K^{*}}_{max}}{q^{2
\bar{B}\rightarrow \rho}_{max}}  \left(\frac{p_{K^{*}}}{p_{\rho}}
\right)_{lim}   \left(\frac{\alpha_{QED}}   {4  \pi}  \right)^{2}
2(C^{2}_{V}+C^{2}_{A})  \frac{[d\Gamma  (\bar{B}\rightarrow  \rho
\ell        \bar{\nu}        )/dq^{2}]_{q^{2}         \rightarrow
q^{2\bar{B}\rightarrow       \rho       }_{max}}}       {[d\Gamma
(\bar{B}\rightarrow   K^{*}  \ell   \bar{\ell}   )/dq^{2}]_{q^{2}
\rightarrow  q^{2\bar{B}\rightarrow K^{*} }_{max}}}  %\nonumber \\ & & 
\label{eqSY17}$$

where $(p_{K^{*}}/p_{\rho})_{lim}=\sqrt{m_{\rho}/m_{K^{*}} }$ and $C_{V}$ and 