  The viscoelastic fluids are usually the blends of a polymeric solute and a
Newtonian solvent. In the presence of a temperature gradient, stratification of
these solutes can take place via the Soret effect. Here, we investigate the
classical Marangoni instability problem for a thin viscoelastic film
considering this binary aspect of the fluid. The film, bounded above by a
deformable free surface, is subjected to heating from below by a solid
substrate. Linear stability analysis performed numerically for perturbations of
finite wavelength (short-wave perturbations) reveals that both monotonic and
oscillatory instabilities can emerge in this system. The interaction between
the thermocapillary and solutocapillary forces in the presence of Soret
diffusion is found to give rise to two different oscillatory instabilities, of
which one mode was overlooked previously, even for the Newtonian binary
mixtures. As a principal result of this work, we provide a complete picture of
the susceptibility to different instability modes based on the model parameter
values. Finally, an approximate model is developed under the framework of
long-wave analysis, which can qualitatively depict the stability behaviour of
the system without numerically solving the problem.
