  Heavy elements are observed in the atmospheres of many DA and DB white
dwarfs, and their presence is attributed to the accretion of matter coming from
debris disks. Several authors have deduced accretion rates from the observed
abundances, taking into account the mixing induced by the convective zones and
the gravitational settling. The obtained values are different for DA and DB
white dwarfs. Here we show that an important process was forgotten in all these
computations: thermohaline mixing, induced by the inverse $\mu$-gradient built
during the accretion process. Taking this mixing into account leads to an
increase of the derived accretion rates, specially for DA white dwarfs, and
modifies the conclusions.
