analysis of ultraviolet spectra for the DBZ star GD40, @juraetal12-1 find a discrepancy between optical and ultraviolet abundances for Si of the same size, but in opposite direction - the abundances are smaller for the optical determinations. Since that study used the same models and atomic data as the one presented here, there is no indication that the atomic data are behind this discrepancy.

  ------------ ---------------- ---------------- ---------------- ---------------- ----------------- ----------------
  Element            PG0843+516       PG1015+161    SDSS1228+1040                                    
                                   Vennes et al.     Melis et al.                                    
  C              $-7.30\pm0.30$         $<-8.00$   $-7.50\pm0.20$   $-6.80\pm0.30$          $<-4.15$         $<-4.85$
  O              $-5.00\pm0.30$   $-5.50\pm0.20$   $-4.55\pm0.20$   $-4.10\pm0.30$    $-3.62\pm0.05$   $-3.68\pm0.10$
  Mg             $-4.90\pm0.20$   $-5.30\pm0.20$   $-5.10\pm0.20$                     $-4.42\pm0.06$   $-4.10\pm0.10$
  Mg (strat)     $-5.00\pm0.20$   $-5.30\pm0.20$   $-5.20\pm0.20$                                    
  Al             $-6.50\pm0.20$                    $-5.75\pm0.20$   $-6.20\pm0.20$                   
  Si             $-5.20\pm0.20$   $-6.40\pm0.20$   $-5.20\pm0.20$   $-4.75\pm0.20$                   
  Si (opt)                                         $-4.70\pm0.20$                     $-4.24\pm0.07$   $-4.35\pm0.11$
  P              $-6.60\pm0.20$                          $<-7.30$   $-7.00\pm0.30$                   
  S              $-5.50\pm0.30$                          $<-6.20$   $-6.60\pm0.20$                   
  Ca                              $-6.30\pm0.20$   $-5.70\pm0.20$                     $-6.11\pm0.04$   $-5.83\pm0.10$
  Ca (strat)                      $-6.45\pm0.20$   $-5.94\pm0.20$                                    
  Cr             $-5.80\pm0.30$         $<-5.80$         $<-6.00$   $-6.10\pm0.30$                     $-5.92\pm0.14$
  Mn                                                                                                   $-6.26\pm0.15$
  Fe             $-4.60\pm0.20$   $-5.50\pm0.30$   $-5.20\pm0.30$   $-4.50\pm0.30$   $ -4.43\pm0.09$   $-4.10\pm0.10$
  Ni             $-6.30\pm0.30$                          $<-6.50$   $-6.70\pm0.30$                           $<-5.60$
  ------------ ---------------- ---------------- ---------------- ---------------- ----------------- ----------------

*Abundance stratification.* Contrary to DB stars like GD40 at similar temperatures, there are no convection zones in the atmospheres and envelopes of our four objects, which would act as a homogeneously mixed reservoir in the 