  The early evolution of planetary systems is expected to depend on various
periods of disk matter accretion onto the central star, which may include the
accretion of metal-rich matter after the star settles on the main sequence.
When this happens, the accreted material is rapidly mixed within the surface
convective zone and induces an inverse mean-molecular-weight gradient, unstable
for thermohaline convection. The induced mixing, which dilutes the metal
excess, may also have important consequences on the light elements abundances.
We model and analyze this process, and present the results according to various
possible accretion scenarios. We give a detailed discussion of the different
ways of treating thermohaline mixing, as proposed by previous authors, and we
converge on a consistent view, including the most recent numerical simulations.
We show how the observations of light elements in stars can be used as tracers
of such events.
