SU(3) breaking in $\overline{B}$/D --> Kl$\overline{\nu}$ decays and value of V$_{cd}$/V$_{cs}$

We determine the SU(3)-breaking effects in the \bar{B}/D \rar Kl\bar \nu decays from the ratio of the \bar{B}/D\rar Kl\bar \nu over the \bar{B}/D \rar \pi l\bar \nu semileptonic form factors using a {\it semi-analytic} expression based on QCD spectral sum rules. We obtain f_+^{\bar{B}\rar K}(0)/f_+^{\bar{B} \rar \pi}(0) = 1.007\pm 0.020 and f_+^{D\rar K}(0)/f_+^{D \rar \pi}(0) = 1.102 \pm 0.007, which combined with the present CLEO data, leads to V_{cd}/V_{cs} = 0.323 \pm 0.058 and then, to: \vert V_{cs}\vert = 0.72\pm 0.10. The prediction on the ratio of mixing angle is (surprisingly) much higher than the value 0.226 \pm 0.004 derived from the unitarity constraint on the CKM matrix, while the corresponding value of V_{cs} is lower. These results can open a window for new physics, though a stringent conclusion needs data with much higher statistics (from e.g tau-charm factory) or/and alternative QCD-based estimates of the ratio of the semileptonic form factors.