  We report on a lattice calculation of the form factors A_0 and A_3 for the
pseudoscalar to vector meson semileptonic decay Bbar^0 to rho^+ l^- nubar_l. We
find that resonant (or pole-type) contributions alone are unable to describe
these two form factors simultaneously. For the quantity A_0(q^2=0), which is
important phenomenologically for the determination of |Vub|, we extract a range
of values, A_0(q^2=0) = (0.16--0.35) +0.09-0.06, where the range is due to
systematic uncertainty and the quoted error is statistical. We have also
determined A_2(q^2=0) = 0.28 +0.09-0.06 +0.04-0.05.
