  We investigate the pseudo-Casimir force acting between two charged surfaces
confining a single polyelectrolyte chain with opposite charge. We expand the
exact free energy to the second order in the local electrostatic field as well
as the replicated polymer density field around the mean-field (saddle-point)
solution. The quadratic terms lead to a fluctuation interaction that is partly
due to the (thermal) Casimir effect for the confined electrostatic field and
partly due to the pseudo-Casimir effect due to the confined replicated polymer
density field. We study the intersurface separation dependence of both effects
and show that the pseudo-Casimir effect leads to a long range attraction
between the surfaces that decays with an anomalous algebraic exponent of $\sim
1.7$, smaller than the standard exponent of 2 in the case of Casimir
interactions.
