Form Factors for $B\to \pi$ and $D\to \pi$ Transitions

Armed with the information on the form factor $\text{fdpi}(0)$ inferred from recent CLEO measurements of SU(3)-breaking effects in charmed meson decays, we have studied the form factor $\text{fbpi}$. In the heavy quark limit, $\text{fbpi}(q^2)$ is related to $\text{fdpi}(q^2)$ in the kinematic region close to zero recoil. Assuming pole dominance for its $q^2$ dependence, $\text{fbpi}(0)$ is estimated to be $\approx 0.39,.$ If the requirement of heavy quark symmetry is relaxed so that it applies only to soft pion emissions from the heavy meson, we find that $\text{fbpi}(0)$ is more likely of order $0.55\sim 0.60\mathrm{.}$