  Collinear velocity relaxation of two spheres immersed in a viscous
incompressible fluid is studied on the basis of an approximate expression for
the retarded hydrodynamic interaction. After a sudden impulse applied to one
sphere, the other one instantaneously starts to move as well, with amplitude
determined by the added mass effect. The velocities of both spheres eventually
decay with a t^{-3/2} long-time tail, but the relative velocity decays with a
t^{-5/2} long-time tail. The three relaxation functions are approximated by
simple expressions involving only a small number of poles in the complex square
root of frequency plane.
