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AA Dor is a close, totally eclipsing, post common-envelope binary with ansdOB-type primary and an extremely low-mass secondary, located close to themass limit of stable central hydrogen burning. Within error limits, it mayeither be a brown dwarf or a late M-type dwarf.  We aim to extract the secondary's contribution to the phase-dependentcomposite spectra. The spectrum and identified lines of the secondary decide onits nature.  In January 2014, we measured the phase-dependent spectrum of AA Dor withXSHOOTER over one complete orbital period. Since the secondary's rotation ispresumable synchronized with the orbital period, its surface strictly dividesinto a day and night side. Therefore, we may obtain the spectrum of its coolside during its transit and of its hot, irradiated side close to itsoccultation. We developed the Virtual Observatory (VO) tool TLISA to search forweak 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 itsoccultation. We identified 57 (20 additional) lines in available UVES(Ultraviolet and Visual Echelle Spectrograph) spectra from 2001. The lines aremostly from C II-III and O II, typical for a low-mass star that is irradiatedand 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 dwarfor an M-type dwarf nature.  Although we identified many emission lines of the secondary's irradiatedsurface, the resolution and signal-to-noise ratio of our UVES and XSHOOTERspectra are not good enough to extract a good spectrum of the secondary'snonirradiated hemisphere.

Search with UVES and XSHOOTER for signatures of the low-mass secondary in the post common-envelope binary AA Dor