  We have derived a new expression for the thermohaline mixing coefficient in
stars, including the effects of radiative levitation and external turbulence,
by solving Boussinesq equations in a quasi-incompressible fluid with a linear
approximation. It is well known that radiative levitation of individual
elements can lead to their accumulation in specific stellar layers. In some
cases, it can induce important effects on the stellar structure. Here we
confirm that this accumulation is moderated by thermohaline convection due to
the resulting inverse $\mu$-gradient. The new coefficient that we have derived
shows that the effect of radiative accelerations on the thermohaline
instability itself is small. This effect must however be checked in all
computations. We also confirm that the presence of large horizontal turbulence
can reduce or even suppress the thermohaline convection. These results are
important as they concern all the cases of heavy element accumulation in stars.
The computations of radiative diffusion have to be revisited including
thermohaline convection and its consequences. It may be one of the basic
reasons for the fact that the observed abundances are always smaller than those
predicted by pure atomic diffusion. In any case, these processes have to
compete with rotation-induced mixing, but this competition is more complex than
previously thought due to their mutual interaction.
