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The $B\to \gamma \ell \nu_\ell$ decay at large energies of the photonreceives a numerically important soft-overlap contribution which is formally ofthe next-to-leading order in the expansion in the inverse photon energy. Wepoint out that this contribution can be calculated within the framework ofheavy-quark expansion and soft-collinear effective theory, making use ofdispersion relations and quark-hadron duality. The soft-overlap contribution isobtained in a full analogy with the similar contribution to the $\gamma^*\gamma \to \pi$ transition form factor. This result strengthens the case forusing the $B\to \gamma \ell \nu_\ell$ decay to constrain the $B$-mesondistribution amplitude and determine its most important parameter, the inversemoment $\lambda_B$.

Soft contribution to $B\to \gamma \ell \nu_\ell$ and the $B$-meson distribution amplitude