  We revisit QCD calculations of radiative heavy meson decay form factors by
including the subleading power corrections from the twist-two photon
distribution amplitude at next-to-leading-order in $\alpha_s$ with the method
of the light-cone sum rules (LCSR). The desired hard-collinear factorization
formula for the vacuum-to-photon correlation function with the interpolating
currents for two heavy mesons is constructed with the
operator-product-expansion technique in the presence of evanescent operators.
Applying the background field approach, the higher twist corrections from both
the two-particle and three-particle photon distribution amplitudes are further
computed in the LCSR framework at leading-order in QCD, up to the twist-four
accuracy. Combining the leading power "point-like" photon contribution at tree
level and the subleading power resolved photon corrections from the newly
derived LCSR, we update theory predictions for the nonperturbative couplings
describing the electromagnetic decay processes of the heavy mesons $H^{\ast \,
\pm} \to H^{\pm} \, \gamma$, $H^{\ast \, 0} \to H^{0} \, \gamma$, $H_s^{\ast \,
\pm} \to H_s^{\pm} \, \gamma$ (with $H=D, \, B$). Furthermore, we perform an
exploratory comparisons of our sum rule computations of the heavy-meson
magnetic couplings with the previous determinations based upon different QCD
approaches and phenomenological models.
