Context : For the spectral analysis of high-resolution and high-signal-to-noise spectra of hot stars , state-of-the-art non-local thermodynamic equilibrium ( NLTE ) model atmospheres are mandatory .
These are strongly dependent on the reliability of the atomic data that is used for their calculation .
In a recent analysis of the ultraviolet ( UV ) spectrum of the DA-type white dwarf G191 $ - $ B2B , 21 Zn iv lines were newly identified .
Because of the lack of Zn iv data , transition probabilities of the isoelectronic Ge vi were adapted for a first , coarse determination of the photospheric Zn abundance .

Aims : Reliable Zn iv and Zn v oscillator strengths are used to improve the Zn abundance determination and to identify more Zn lines in the spectra of G191 $ - $ B2B and the DO-type white dwarf RE 0503 $ - $ 289 .

Methods : We performed new calculations of Zn iv and Zn v oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of the Zn iv - v spectrum exhibited in high-resolution and high-S/N UV observations of G191 $ - $ B2B and RE 0503 $ - $ 289 .

Results : In the UV spectrum of G191 $ - $ B2B , we identify 31 Zn iv and 16 Zn v lines .
Most of these are identified for the first time in any star .
We can reproduce well almost all of them at \log \mathrm { Zn } = -5.52 \pm 0.2 ( mass fraction , about 1.7 times solar ) .
In particular , the Zn iv / Zn v ionization equilibrium , which is a very sensitive T _ { \mathrm { eff } } indicator , is well reproduced with the previously determined \mbox { $T _ { \mathrm { eff } } $ } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 60 000 \pm 200 % 0 \mathrm { K } and \log g \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 7.60 \pm 0.05 .
In the spectrum of RE 0503 $ - $ 289 , we identified 128 Zn v lines for the first time and determined \log \mathrm { Zn } = -3.57 \pm 0.2 ( 155 times solar ) .

Conclusions : Reliable measurements and calculations of atomic data are a pre-requisite for stellar-atmosphere modeling .
Observed Zn iv and Zn v line profiles in two white dwarf ( G191 $ - $ B2B and RE 0503 $ - $ 289 ) ultraviolet spectra were well reproduced with our newly calculated oscillator strengths .
This allowed us to determine the photospheric Zn abundance of these two stars precisely .