  The effects of polymer additives on Rayleigh--Taylor (RT) instability of
immiscible fluids is investigated using the Oldroyd-B viscoelastic model.
Analytic results obtained exploiting the phase-field approach show that in
polymer solution the growth rate of the instability speeds up with elasticity
(but remains slower than in the pure solvent case). Numerical simulations of
the viscoelastic binary fluid model confirm this picture.
