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LB 3459 (AA Dor) is an eclipsing, close, post common-envelope binaryconsisting of an sdOB primary star and an unseen secondary with anextraordinarly low mass - formally a brown dwarf. A recent NLTE spectralanalysis shows a discrepancy with the surface gravity, which is derived fromanalyses of radial-velocity and lightcurves. We aim at precisely determing ofthe photospheric parameters of the primary, especially of the surface gravity,and searching for weak metal lines in the far UV. We performed a detailedspectral analysis of the far-UV spectrum of LB 3459 obtained with FUSE by meansof state-of-the-art NLTE model-atmosphere techniques.  A strong contamination of the far-UV spectrum of LB 3459 by interstellar lineabsorption hampers a precise determination of the photospheric properties ofits primary star. Its effective temperature (42 kK) was confirmed by theevaluation of new ionization equilibria. For the first time, phosphorus andsulfur have been identified in the spectrum of LB 3459. Their photosphericabundances are solar and 0.01 times solar, respectively. From the C III1174-1177A multiplet, we can measure the rotational velocity of 35 +/- 5 km/secof the primary of LB 3459 and confirm that the rotation is bound. From are-analysis of optical and UV spectra, we determine a higher log g = 5.3 (cgs)that reduces the discrepancy in mass determination in comparison to analyses ofradial-velocity and lightcurves. However, the problem is not completely solved.

FUSE spectroscopy of sdOB primary of the post common-envelope binary LB 3459 (AA Dor)