  We report the first detection of cesium (Z = 55) in the atmosphere of a white
dwarf. Around a dozen absorption lines of Cs IV, Cs V, and Cs VI have been
identified in the Far Ultraviolet Spectroscopic Explorer spectrum of the
He-rich white dwarf HD 149499B (Teff = 49,500 K, log g = 7.97). The lines have
equivalent widths ranging from 2.3 to 26.9 m\r{A}. We performed a spectral
synthesis analysis to determine the cesium content in the atmosphere. Non-LTE
atmosphere models were computed by considering cesium explicitly in the
calculations. For this purpose we calculated oscillator strengths for the
bound-bound transitions of Cs IV-Cs VI with both AUTOSTRUCTURE
(multiconfiguration Breit-Pauli) and GRASP2K (multiconfiguration Dirac-Fock)
atomic structure codes as neither measured nor theoretical values are reported
in the literature. We determined a cesium abundance of log N(Cs)/N(He) =
-5.45(0.35), which can also be expressed in terms of the mass fraction log
X(Cs) = -3.95(0.35).
