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Astron. Astrophys. 354, L49-L52 (2000)


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Letter to the Editor

Evidence for a substellar secondary in the magnetic cataclysmic binary EF Eridani *

K. Beuermann 1, P. Wheatley 2, G. Ramsay 3, F. Euchner 1 and B.T. Gänsicke 1

1 Universitäts-Sternwarte, Geismarlandstrasse 11, 37083 Göttingen, Germany
2 Leicester University, X-ray Astronomy Group, Department of Physics and Astronomy, University Road, Leicester LE1 7RH, UK
3 University College of London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

Received 6 December 1999 / Accepted 21 December 1999

Abstract

Low-state spectrophotometry of the short-period polar EF Eridani ([FORMULA] = 81 min) found the system at [FORMULA] with no trace of the companion (Wheatley and Ramsay 1998). We show that the lack of such spectral features implies that the companion to the white dwarf in EF Eri has a spectral type later than M9 and is either a transition object at the brink of hydrogen burning or a brown dwarf. The optical low state spectrum indicates a temperature of the white dwarf of [FORMULA][FORMULA] K. This is one of the coldest white dwarfs in cataclysmic variables, implying a cooling age [FORMULA] yrs or accretional heating at a rate as given by gravitational radiation. The large age of the system excludes a warm brown dwarf as companion. EF Eri has either just passed through the period minimum of cataclysmic variable stars or has started mass transfer from an old brown dwarf secondary.

Key words: stars: evolution – stars: formation – stars: low-mass, brown dwarfs – stars: novae, cataclysmic variables – stars: white dwarfs

* Based in part on observations at the European Southern Observatory La Silla (Chile)

Send offprint requests to: beuermann@uni-sw.gwdg.de

© European Southern Observatory (ESO) 2000

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