AA Dor is a close, totally eclipsing, post common-envelope binary with an sdOB-type primary and an extremely low-mass secondary, located close to the mass limit of stable central hydrogen burning. Within error limits, it may either be a brown dwarf o
r a late M-type dwarf. We aim to extract the secondarys contribution to the phase-dependent composite spectra. The spectrum and identified lines of the secondary decide on its nature. In January 2014, we measured the phase-dependent spectrum of AA Dor with XSHOOTER over one complete orbital period. Since the secondarys rotation is presumable synchronized with the orbital period, its surface strictly divides into a day and night side. Therefore, we may obtain the spectrum of its cool side during its transit and of its hot, irradiated side close to its occultation. We developed the Virtual Observatory (VO) tool TLISA to search for weak lines of a faint companion in a binary system. We identified 53 spectral lines of the secondary in the ultraviolet-blue, visual, and near-infrared XSHOOTER spectra that are strongest close to its occultation. We identified 57 (20 additional) lines in available UVES (Ultraviolet and Visual Echelle Spectrograph) spectra from 2001. The lines are mostly from C II-III and O II, typical for a low-mass star that is irradiated and heated by the primary. We verified the orbital period of P = 22597.033201 +/- 0.00007 s and determined the orbital velocity Ksec = 232.9 (+16.6 / -6.5) km/s of the secondary. The mass of the secondary is Msec = 0.081 (+0.018 / -0.010) Msun and, hence, it is not possible to reliably determine a brown dwarf or an M-type dwarf nature. Although we identified many emission lines of the secondarys irradiated surface, the resolution and signal-to-noise ratio of our UVES and XSHOOTER spectra are not good enough to extract a good spectrum of the secondarys nonirradiated hemisphere.
For the spectral analysis of spectra of hot stars with a high resolution and high signal-to-noise ratio (S/N), advanced non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the
atomic data that are used for their calculation. Reliable Xe VI oscillator strengths are used to identify Xe lines in the ultraviolet spectrum of the DO-type white dwarf RE0503-289 and to determine its photospheric Xe abundance. We publish newly calculated oscillator strengths that are based on a recently measured Xe VI laboratory line spectrum. These strengths were used to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models to analyze Xe VI lines exhibited in high-resolution and high S/N UV observations of RE0503-289. We identify three hitherto unknown Xe VI lines in the ultraviolet spectrum of RE0503-289 and confirm the previously measured photospheric Xe abundance of this white dwarf (log Xe = -4.2 +/- 0.6). Reliable measurements and calculations of atomic data are prerequisite for stellar-atmosphere modeling. Observed Xe VI line profiles in the ultraviolet spectrum of the white dwarf RE0503-289 were well reproduced with the newly calculated Xe VI oscillator strengths.
