Pseudoscalar decay constantsfFandfDfrom exclusiveBdecays

We show that ‖$V_{\mathrm{cs}}$ ${\Vert }^2$ $f_F$ ${\mathrm{}}^2$ [or ‖$V_{\mathrm{cd}}$ ${\Vert }^2$ $f_D$ ${\mathrm{}}^2$] can be obtained with good accuracy from the ratio between Γ($B^{\mathrm{-}}$→$F^{\mathrm{-}}$ $D^0$) [or Γ($B^{\mathrm{-}}$→$D^{\mathrm{-}}$ $D^0$)] and ${\Gamma }_{\Delta }$($B^{\mathrm{-}}$→$l^{\mathrm{-}}$ ν¯$D^0$), where $l^{\mathrm{-}}$=e or μ and ${\Gamma }_{\Delta }$ means the decay rate with appropriate kinematical cuts on &+${\mathrm{P}}_{\nu \mathrm{¯}}$ ${)}^2$. We also note that Γ(${\mathit{B}}^{\mathrm{-}}$→${\mathit{F}}^{\mathrm{-}}$ ${\mathit{D}}^{\mathrm{O}}$)/Γ(${\mathit{B}}^{\mathrm{-}}$→${\mathit{D}}^{\mathrm{-}}$ ${\mathit{D}}^{\mathrm{O}}$) determines ‖${\mathit{V}}_{\mathit{c}\mathit{s}}$ ${\mathrm{\Vert }}^2$ ${\mathit{f}}_{\mathit{F}}^2$/‖${\mathit{V}}_{\mathit{c}\mathit{d}}$ ${\mathrm{\Vert }}^2$ ${\mathit{f}}_{\mathit{D}}^2$ with only a few percent uncertainty. Thus a precise measurement of the above decay rates would provide us with a reliable determination of the pseudoscalar decay constants $f_F$ and $f_D$.