accretion/diffusion scenario. Assuming a steady state between the two processes, we thus expect a stratified abundance configuration. Whether this can explain the observations will be studied in Sect.\[s-diffusion\].

*Genuine variation of the accretion rates.* As will also be discussed in the next section, the time scales for diffusion in these atmospheres are of the order of days. If the accretion rate is not constant the observed abundances may change on the same short time scales. Given that the COS and ground-based observations that we analysed were taken months to years apart, such variations can not be excluded. Noticeable variations of the [Ca]{} equivalent widths in the debris disc white dwarf G29-38 were reported by @vonhippeletal07-2. However, a similar study on the same star by @debes+lopez-morales08-1 did not find any variations in the line strengths. Thus, the current evidence for accretion rate variations on time scales of months to years is ambiguous, and a second-epoch COS observations of the stars studied here would be desirable.

We also noticed an unidentified absorption feature between 1400 and 1410 Å, with a strength roughly correlated with the Si abundances. Such a feature has been discussed in the literature and related to an autoionisation line 