  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.
  To search for Zr and Xe lines in the ultraviolet (UV) spectra of G191-B2B and
RE0503-289, new Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths were
calculated. This allows for the first time, determination of the Zr abundance
in white dwarf (WD) stars and improvement of the Xe abundance determinations.
  We calculated Zr IV-VII, Xe IV-V, and Xe VIII oscillator strengths to
consider radiative and collisional bound-bound transitions of Zr and Xe in our
NLTE stellar-atmosphere models for the analysis of their lines exhibited in UV
observations of the hot WDs G191-B2B and RE0503-289.
  We identified one new Zr IV, 14 new Zr V, and ten new Zr VI lines in the
spectrum of RE0503-289. Zr was detected for the first time in a WD. We measured
a Zr abundance of -3.5 +/- 0.2 (logarithmic mass fraction, approx. 11 500 times
solar). We dentified five new Xe VI lines and determined a Xe abundance of -3.9
+/- 0.2 (approx. 7500 times solar). We determined a preliminary photospheric Al
abundance of -4.3 +/- 0.2 (solar) in RE0503-289. In the spectra of G191-B2B, no
Zr line was identified. The strongest Zr IV line (1598.948 A) in our model gave
an upper limit of -5.6 +/- 0.3 which is about 100 times solar. No Xe line was
identified in the UV spectrum of G191-B2B and we confirmed the previously
determined upper limit of -6.8 +/- 0.3 (ten times solar).
  Precise measurements and calculations of atomic data are a prerequisite for
advanced NLTE stellar-atmosphere modeling. Observed Zr IV - VI and Xe VI - VII
line profiles in the UV spectrum of RE0503-289 were simultaneously well
reproduced.
