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For the spectral analysis of high-resolution and high-signal-to-noise spectraof hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE)model-atmospheres are mandatory. These are strongly dependent on thereliability of the atomic data that is used for their calculation. In a recentanalysis 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.  We performed new calculations of Zn IV and Zn V oscillator strengths toconsider their radiative and collisional bound-bound transitions in detail inour NLTE stellar-atmosphere models for the analysis of the Zn IV - V spectrumexhibited in high-resolution and high-S/N UV observations of G191-B2B andRE0503-289.  In the UV spectrum of G191-B2B, we identify 31 Zn IV and 16 Zn V lines. Mostof these are identified for the first time in any star. We can reproduce wellalmost all of them at log Zn = -5.52 +/- 0.2 (mass fraction, about 1.7 timessolar). In particular, the Zn IV / Zn V ionization equilibrium, which is a verysensitive indicator for the effective temperature, is well reproduced with thepreviously determined Teff = 60000 +/- 2000 and log g = 7.60 +/- 0.05. In thespectrum of RE0503-289, we identified 128 Zn V lines for the first time anddetermined log Zn = -3.57 +/- 0.2 (155 times solar).  Reliable measurements and calculations of atomic data are a pre-requisite forstellar-atmosphere modeling. Observed Zn IV and Zn V line profiles in two whitedwarf (G191-B2B and RE0503-289) ultraviolet spectra were well reproduced withour newly calculated oscillator strengths. This allowed us to determine thephotospheric Zn abundance of these two stars precisely.

Stellar laboratories II. New Zn IV and Zn V oscillator strengths and their validation in the hot white dwarfs G191-B2B and RE0503-289