is equal to the accretion rate polluting the atmosphere. The constant diffusion flux then in turn allows the determination of the abundance stratification $X(\tau)$ [see also @vennesetal11-1 for a thorough discussion].

We calculated new stratified models and synthetic spectra for all objects, using the steady state condition and the abundances (at $\tau
= 2/3$) from Table \[t-abundances\]. The resulting spectra are almost indistinguishable from those of the homogeneous atmospheres; the only exception are small increases of the optical [Mg]{} and [Ca]{} line strengths. The small change can easily be explained by the structure of the stratified atmosphere. In these models $\rho
\,v$ increases with depth, and consequently the abundance decreases. On the other hand a monochromatic optical depth of $\approx
2/3$ is reached in the ultraviolet near Rosseland optical depth of $\tau_\mathrm{Ross}\simeq2/3$, while it is reached at $\tau_\mathrm{Ross} \approx 0.15$ for $\lambda = 4480$ Å, i.e. higher in the atmosphere, where the abundance is correspondingly higher.

For PG0843+516, PG1015+161, and SDSS1228+1040, the Ca and Mg abundances were obtained from the optical data (Sect.\[s-optobs\]) and our models. We have iterated them by fitting to stratified models (denoted with “strat” in Table \[t-abundances\]). For GALEX1931+0117, we adopted the photospheric Mg an Ca 