$C_{A}$ are the QCD corrected Wilson coefficients. The matrix element $|V_{ub}|$ may be determined if the RHS can be obtained by experiment and $|V_{ts}|$ from the unitarity condition. The problem is that in the zero recoil limit $p_{\rho
, K^{*}}\rightarrow 0 $, i.e., $q^{2}=q^{2}_{max}$, the $q^{2}$ distributions vanish due to the phase space suppression. This means that experimentally there should be no events at that point and very few in the neighborhood, making it a very difficult measurement.

Dib and Vera [@DV] relate $B  \rightarrow  \rho \, \ell \,
\bar{\nu}$ to $ D \rightarrow  \rho \, \ell \, \bar{\nu}$ also at the point of zero recoil, using the heavy quark symmetry, to get a model independent result to leading order in inverse powers of large masses. At the kinematical point of zero recoil, $y=1$, where $y=(m^{2}_{I} + m^{2}_{\rho} - q^{2})/2m_{I}  m_{\rho}$ and $I=B$ or $D$ they find

$$\begin{aligned}
\frac{d\Gamma  (B\rightarrow  \rho e \nu  )/dy}
{d\Gamma (D \rightarrow  \rho e \nu )/dy}  \begin{array}{l}  \mid
\\[-0.3cm]  \mid  _{y\rightarrow  1} \end{array} & = & \frac{\mid
V_{ub}\mid^{2}}{\mid            V_{cd}\mid^{2}}            \left(
\frac{f^{[B]}_{A_{1}}(1)}{f^{[D]}_
{A_{1}}(1)}\right)^{2}\left(\frac{m_{B}-m_{\rho}}{m_{D}-m_{\rho}}
\right)^{2} \label{eqDV6} \end{aligned}$$

They determine the ratio of form factors $f^{[B]}_{A_{1}}(1)/f^{[D]}_{A_{1}}(1)$ in the constituent quark model at the tree-level in the HQET and 