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For the spectral analysis of high-resolution and high-signal-to-noise (S/N)spectra of hot stars, advanced non-local thermodynamic equilibrium (NLTE) modelatmospheres are mandatory. These atmospheres are strongly dependent on thereliability 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 theDA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and todetermine their photospheric Ga abundances. We newly calculated Ga IV - VIoscillator strengths to consider their radiative and collisional bound-boundtransitions in detail in our NLTE stellar-atmosphere models for analyzing of Galines exhibited in high-resolution and high-S/N UV observations of G191-B2B andRE0503-289. We unambiguously detected 20 isolated and 6 blended (with lines ofother species) Ga V lines in the Far Ultraviolet Spectroscopic Explorer (FUSE)spectrum of RE0503-289. The identification of Ga IV and Ga VI lines isuncertain because they are weak and partly blended by other lines. Thedetermined Ga abundance is 3.5 +/- 0.5 x 10**-5 (mass fraction, about 625 timessolar). The Ga IV / GA V ionization equilibrium, which is a very sensitiveindicator for the effective temperature, is well reproduced in RE0503-289. Weidentified the strongest Ga IV lines (1258.801, 1338.129 A) in the HST/STIS(Hubble Space Telescope / Space Telescope Imaging Spectrograph) spectrum ofG191-B2B and measured a Ga abundance of 2.0 +/- 0.5 x 10**-6 (about 22 timessolar). Reliable measurements and calculations of atomic data are aprerequisite for stellar-atmosphere modeling. Observed Ga IV - V line profilesin two white dwarf (G191-B2B and RE0503-289) ultraviolet spectra were wellreproduced with our newly calculated oscillator strengths. For the first time,this allowed us to determine the photospheric Ga abundance in white dwarfs.

Stellar laboratories IV. New Ga IV, Ga V, and Ga VI oscillator strengths and the gallium abundance in the hot white dwarfs G191-B2B and RE0503-289