  We report the first systematic analysis of the off-light-cone effects in sum
rules for heavy-to-light form factors. These effects are investigated in a
model based on scalar constituents, which allows a technically rather simple
analysis but has the essential features of the analogous QCD calculation. The
correlator relevant for the extraction of the heavy-to-light form factor is
calculated in two different ways: first, by adopting the full Bethe-Salpeter
amplitude of the light meson and, second, by performing the expansion of this
amplitude near the light cone $x^2=0$. We demonstrate that the contributions to
the correlator from the light-cone term $x^2=0$ and the off-light-cone terms
$x^2\ne 0$ have the same order in the $1/m_Q$ expansion. The light-cone
correlator, corresponding to $x^2=0$, is shown to systematically overestimate
the full correlator, the difference being $\sim \Lambda_{\rm QCD}/\delta$, with
$\delta$ the continuum subtraction parameter of order 1 GeV. Numerically, this
difference is found to be 10-20%.
