  We calculate the effective force mediated by thermally excited capillary
waves between spherical or disklike colloids trapped at a fluid interface. This
Casimir type interaction is shown to depend sensitively on the boundary
conditions imposed at the three-phase contact line. For large distances between
the colloids an unexpected cancellation of attractive and repulsive
contributions is observed leading to a fluctuation force which decays
algebraically very rapidly. For small separations the resulting force is rather
strong and it may play an important role in two-dimensional colloid aggregation
if direct van der Waals forces are weak.
