  While the metallicity excess observed in the central stars of planetary
systems is confirmed by all recent observations, the reason for this excess is
still a subject of debate : is it primordial, is it the result of accretion or
both? The basic argument against an accretion origin is related to the masses
of the outer convective zones which vary by more than one order of magnitude
among the considered stars while the observed overabundances of metals are
similar. We show here that in previous discussions a fundamental process was
forgotten : thermohaline convection induced by the inverse mu-gradient.
"Metallic fingers" may be created which dilute the accreted matter inside the
star. Introducing this effect may reconcile the overabundances expected in case
of accretion with the observations in stars of different masses.
