2000A&A...356..665R

We present a spectral analysis of the sdOB primary star of the binary system LB 3459 based on high-resolution high-S/N optical and UV spectra. The metal abundances are determined by means of state-of-the-art NLTE model atmospheres. We determined T_eff=42kK and logg=5.2 within very small error limits. The He (1/125 solar), C (1/265), N (1/33), O (1/12), and Si (1/5) abundances appear strongly depleted while that of Fe and Ni are roughly solar and Mg is strongly enriched by a factor of 6. The spectroscopic distance to LB 3459 is d=396pc. The mass of the primary component of LB 3459 is 0.330M_☉ derived from comparisons with theoretical models for sdO stars in the logT_eff- logg plane. The mass of the secondary is then 0.066M_☉ derived from the mass function. There remains some disagreement between the radius derived from logg and the above mass, and that derived from analysis of the radial-velocity curve and the eclipse curves. LB 3459 is a close binary system which had experienced a common envelope (CE) phase during its evolution. It fits in the ``low mass case B'' scenario of Iben & Livio (1993PASP..105.1373I) and the secondary is a brown dwarf. The spectroscopically determined rotational velocity of the primary is v_rot=34±10km/s. Thus even bound rotation (v_rot=45.7km/s) cannot be ruled out.