  We analyze the effective potential for nanoparticles trapped at a fluid
interface within a simple model which incorporates surface and line tensions as
well as a thermal average over interface fluctuations (capillary waves). For a
single colloid, a reduced steepness of the potential well hindering movements
out of the interface plane compared to rigid interface models is observed, and
an instability of the capillary wave partition sum in case of negative line
tensions is pointed out. For two colloids, averaging over the capillary waves
leads to an effective Casimir--type interaction which is long--ranged,
power-like in the inverse distance but whose power sensitively depends on
possible restrictions of the colloid degress of freedom. A nonzero line tension
leads to changes in the magnitude but not in the functional form of the
effective potential asymptotics.
