  For the spectral analysis of high-resolution and high-signal-to-noise (S/N)
spectra of hot stars, advanced non-local thermodynamic equilibrium (NLTE) model
atmospheres are mandatory. These atmospheres are strongly dependent on the
reliability of the atomic data that are used to calculate them. Reliable Ga IV
- VI oscillator strengths are used to identify Ga 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 Ga abundances. We newly calculated Ga IV - VI
oscillator strengths to consider their radiative and collisional bound-bound
transitions in detail in our NLTE stellar-atmosphere models for analyzing of Ga
lines exhibited in high-resolution and high-S/N UV observations of G191-B2B and
RE0503-289. We unambiguously detected 20 isolated and 6 blended (with lines of
other species) Ga V lines in the Far Ultraviolet Spectroscopic Explorer (FUSE)
spectrum of RE0503-289. The identification of Ga IV and Ga VI lines is
uncertain because they are weak and partly blended by other lines. The
determined Ga abundance is 3.5 +/- 0.5 x 10**-5 (mass fraction, about 625 times
solar). The Ga IV / GA V ionization equilibrium, which is a very sensitive
indicator for the effective temperature, is well reproduced in RE0503-289. We
identified the strongest Ga IV lines (1258.801, 1338.129 A) in the HST/STIS
(Hubble Space Telescope / Space Telescope Imaging Spectrograph) spectrum of
G191-B2B and measured a Ga abundance of 2.0 +/- 0.5 x 10**-6 (about 22 times
solar). Reliable measurements and calculations of atomic data are a
prerequisite for stellar-atmosphere modeling. Observed Ga IV - V line profiles
in two white dwarf (G191-B2B and RE0503-289) ultraviolet spectra were well
reproduced with our newly calculated oscillator strengths. For the first time,
this allowed us to determine the photospheric Ga abundance in white dwarfs.
