Abstract :
We study the form factor f+H (H=D, B) involved in H–π transitions. We use data on D0→π−l+ν to find fD+ at low recoil. Then we use perturbative QCD methods to calculate at larger recoil the contributions to f+H that break heavy quark symmetry (HQS). Using HQS relations we obtain an estimate of fB+ which includes first order corrections in 1/(2mQ). Comparison with recent data on B0→π−l+ν gives Vub=0.0025±0.0005ex±0.0006th
