the white dwarf photosphere if the accretion rate varies on shorter time scales [@koester09-1]. While the life times of the debris discs are subject to large uncertainties, there are theoretical [@rafikov11-2; @metzgeretal12-1] and observational (@girvenetal12-1, Farihi et al. 2012 in press) arguments that suggest that the accretion rates onto the white dwarfs may vary significantly over periods that are short compared to the diffusion time scales. In fact, some of the most heavily polluted white dwarfs have no infrared excess [@farihietal09-1; @kleinetal11-1], and may have accreted all the circumstellar debris a few diffusion time scales ago [@farihietal09-1; @girvenetal12-1].

We are currently carrying out an ultraviolet spectroscopic survey of young DA white dwarfs that have cooling ages of 20 to 200Myr, metal sinking time scales of a few days, and are hence guaranteed to be in accretion-diffusion equilibrium. The aim of this survey is to determine the fraction of white dwarfs that are presently accreting planetary debris, and to determine accurate abundances for a subset. Here we present the analysis of four heavily polluted white dwarfs that are known to also host planetary debris discs.

![image](4daz_f1.ps){width="2\columnwidth"}

Observations
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The targets for our ongoing far-ultraviolet spectroscopic survey of young and correspondingly warm 