Relationship between form factors in semileptonic bar B and D decays and exclusive rare bar B-meson decays

We show that existing data on the semileptonic decays D→Ke+νe and D→K*e+νe provide information on matrix elements relevant for the rare bar B-meson decays bar B→Ke+e-, bar B→K*e+e-, and bar B→K*γ and that future data on bar B→ρe bar νe and bar B→πe bar νe will further constrain them. A discussion of how the form factors for these latter decays are in turn determined by semileptonic D decays at the corresponding recoil momenta is also presented