  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.
  To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type
white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to determine
their photospheric Mo abundances, newly calculated Mo IV - VII oscillator
strengths are used.
  We identified twelve Mo V and nine Mo VI lines in the UV spectrum of
RE0503-289 and measured a photospheric Mo abundance of 1.2 - 3.0 x 10**-4 (mass
fraction, 22500 - 56400 times the solar abundance). In addition, from the As V
and Sn IV resonance lines, we measured mass fractions of arsenic (0.5 - 1.3 x
10**-5, about 300 - 1200 times solar) and tin (1.3 - 3.2 x 10**-4, about 14300
35200 times solar). For G191-B2B, upper limits were determined for the
abundances of Mo (5.3 x 10**-7, 100 times solar) and, in addition, for Kr (1.1
x 10**-6, 10 times solar) and Xe (1.7 x 10**-7, 10 times solar). The arsenic
abundance was determined (2.3 - 5.9 x 10**-7, about 21 - 53 times solar). A
new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS,
has been constructed to provide weighted oscillator strengths and transition
probabilities.
  Reliable measurements and calculations of atomic data are a prerequisite for
stellar-atmosphere modeling. Observed Mo V - VI line profiles in the UV
spectrum of the white dwarf RE0503-289 were well reproduced with our newly
calculated oscillator strengths. For the first time, this allowed to determine
the photospheric Mo abundance in a white dwarf.
