  For the spectral analysis of high-resolution and high-signal-to-noise (S/N)
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. Reliable Ba
V - VII oscillator strengths are used to identify Ba lines in the spectra of
the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to
determine their photospheric Ba abundances. We newly calculated Ba V - VII
oscillator strengths to consider their radiative and collisional bound-bound
transitions in detail in our NLTE stellar-atmosphere models for the analysis of
Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B
and RE0503-289. For the first time, we identified highly ionized Ba in the
spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far
Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI /
Ba VII ionization equilibrium is well reproduced with the previously determined
effective temperature of 70000 K and surface gravity of $\log g = 7.5$. The Ba
abundance is $3.5 \pm 0.5 \times 10^{-4}$ (mass fraction, about 23000 times the
solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba
VII line (at 993.41 \AA) only, and determined a Ba abundance of $4.0 \pm 0.5
\times 10^{-6}$ (about 265 times solar). Reliable measurements and calculations
of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba
VI - VII line profiles in two white dwarfs' (G191-B2B and RE0503-289)
far-ultraviolet spectra were well reproduced with our newly calculated
oscillator strengths. This allowed to determine the photospheric Ba abundance
of these two stars precisely.
