  The asymptotic behavior of the optical potential, describing elastic
scattering of a charged particle $\alpha$ off a bound state of two charged, or
one charged and one neutral, particles at small momentum transfer
$\Delta_{\alpha}$ or equivalently at large intercluster distance
$\rho_{\alpha}$, is investigated within the framework of the exact three-body
theory. For the three-charged-particle Green function that occurs in the exact
expression for the optical potential, a recently derived expression, which is
appropriate for the asymptotic region under consideration, is used. We find
that for arbitrary values of the energy parameter the non-static part of the
optical potential behaves for $\Delta_{\alpha} \rightarrow 0$ as
$C_{1}\Delta_{\alpha} + o\,(\Delta_{\alpha})$. From this we derive for the
Fourier transform of its on-shell restriction for $\rho_{\alpha} \rightarrow
\infty$ the behavior $-a/2\rho_{\alpha}^4 + o\,(1/\rho_{\alpha}^4)$, i.e.,
dipole or quadrupole terms do not occur in the coordinate-space asymptotics.
This result corroborates the standard one, which is obtained by perturbative
methods. The general, energy-dependent expression for the dynamic
polarisability $C_{1}$ is derived; on the energy shell it reduces to the
conventional polarisability $a$ which is independent of the energy. We
emphasize that the present derivation is {\em non-perturbative}, i.e., it does
not make use of adiabatic or similar approximations, and is valid for energies
{\em below as well as above the three-body dissociation threshold}.
