Context : LB 3459 ( AA Dor ) is an eclipsing , close , post common-envelope binary ( PCEB ) consisting of an sdOB primary star and an unseen secondary with an extraordinarily low mass ( M _ { 2 } \approx 0.066 \mathrm { M _ { \odot } } ) – formally a brown dwarf .
A recent NLTE spectral analysis shows a discrepancy with the surface gravity , which is derived from analyses of radial-velocity and lightcurves .

Aims : We aim at precisely determining of the photospheric parameters of the primary , especially of the surface gravity , and searching for weak metal lines in the far UV .

Methods : We performed a detailed spectral analysis of the far-UV spectrum of LB 3459 obtained with FUSE by means of state-of-the-art NLTE model-atmosphere techniques .

Results : A strong contamination of the far-UV spectrum of LB 3459 by interstellar line absorption hampers a precise determination of the photospheric properties of its primary star .
Its effective temperature ( \mbox { $T _ { \mathrm { eff } } $ } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 42 \mathrm { kK } ) was confirmed by the evaluation of new ionization equilibria .
For the first time , phosphorus and sulfur have been identified in the spectrum of LB 3459 .
Their photospheric abundances are solar and 0.01 times solar , respectively .
From the C III \lambda \lambda 1174 - 1177 Å multiplet , we can measure the rotational velocity v _ { \mathrm { rot } } = 35 \pm 5 \mathrm { km / sec } of the primary of LB 3459 and confirm that the rotation is bound .
From a re-analysis of optical and UV spectra ( analogue to Rauch 2000 ) , we determine a slightly higher surface gravity \log g \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 5.3 \pm 0.1 compared to Rauch ( 2000 , \log g \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 5.2 \pm 0.1 ) .

Conclusions : The rotational velocity of the primary of LB 3459 is consistent with a bound rotation .
The higher \log g reduces the discrepancy in mass determination in comparison to analyses of radial-velocity and lightcurves .
However , the problem is not completely solved .