  We study the form factors for a heavy meson into the S-wave $K\pi/\pi\pi$
system with an invariant mass below 1~GeV. The mesonic final state interactions
are described in terms of the scalar form factors, which are obtained from
unitarized chiral perturbation theory. Employing generalized light-cone
distribution amplitudes, we compute the heavy-to-light transition using
light-cone sum rules. Our approach simultaneously respects constraints from
analyticity and unitarity, and also takes advantage of the power expansion in
the $1/m_b$ and the strong coupling constant.
