Recent Investigations on AA Doradus

AA Dor is an eclipsing, post common-envelope binary with an sdOB-type primary and an unseen low-mass secondary, believed to be a brown dwarf. Eleven years ago, a NLTE spectral analysis of the primary showed a discrepancy with the surface gravity that was derived by radial-velocity and light-curve analysis that could not be explained. Since then, emission lines of the secondary were identified in optical spectra and its orbital-velocity amplitude was measured. Thus, the masses of both components are known, however, within relatively large error ranges. The secondary's mass was found to be around the stellar hydrogen-burning mass limit and, thus, it may be a brown dwarf or a late M-type dwarf. In addition, a precise determination of the primary's rotational velocity showed recently, that it rotates at about 65% of bound rotation - much slower than previously assumed. A new spectral analysis by means of metal-line blanketed, state-of-the-art, non-LTE model atmospheres solves the so-called gravity problem in AA Dor - our result for the surface gravity is, within the error limits, in agreement with the value from light-curve analysis. We present details of our recent investigations on AA Dor.

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