Comment on Vub from exclusive semileptonic B and D decays

The prospects for determining |Vub| from exclusive B semileptonic decay are discussed. The double ratio of form factors (f(B→ρ)/f(B→K∗))/(f(D→ρ)/f(D→K∗)) is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from B→ρ ℓ ν̄ℓ, B→K∗ℓ ℓ̄ and D→ρ ℓ̄ νℓ can lead to a model independent determination of |Vub| with an uncertainty from theory of about 10%.