  The presence of silicon at the surface of the two Hyades DA white dwarfs WD
0421+162 and WD 0431+126 requires mechanisms that counteract the effects of the
downward diffusion. Radiative levitation calculations indicate that the silicon
abundance observed in WD 0431+126 corresponds to the abundance supported by
radiative levitation. Detailed time-dependent diffusion calculations that take
into account radiative levitation and accretion indicate that accretion with
rates of dM/dt(Si) < 1.00E4 g/s could also be present without disrupting the
abundance supported by radiative levitation. In the case of WD 0421+162,
accretion with a rate of dM/dt(Si) = 1.25E5 g/s must be invoked, because its
observed silicon abundance is larger than the abundance supported by radiative
levitation. This accretion rate is lower than the accretion rate given by the
accretion-diffusion model, because the radiative levitation slows down the
downward diffusion of silicon. The silicon abundances observed in the two
Hyades white dwarfs cannot be interpreted solely in terms of accretion. The
interpretation of the silicon abundances must take into account the interplay
between radiative levitation and accretion.
