HS 1857+5144: a hot and young pre-cataclysmic variable

¹ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK e-mail: A.Aungwerojwit@warwick.ac.uk
² Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁴ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Institute of Astronomy and Astrophysics, National Observatory of Athens, PO Box 20048, Athens 11810, Greece
⁶ McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712, USA

Received: 10 February 2007
Accepted: 11 April 2007

Abstract

Aims.We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).

Methods.Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).

Results.The light curves of HS 1857+5144 display a sinusoidal variation with a period of P_orb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1 mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of 0.6-1.0  and 70 000-100 000 K, respectively. The detection of 4686 absorption classifies the primary star of HS 1857+5144 as a DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be 0.15-0.30  and 45°-55°, respectively.

Conclusions.We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ~10⁵ yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap.