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Issue A&A
Volume 379, Number 3, December I 2001
Page(s) 893 - 904
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011376

A&A 379, 893-904 (2001)
DOI: 10.1051/0004-6361:20011376

HS 0705+6700: A new eclipsing sdB binary

H. Drechsel1, U. Heber1, R. Napiwotzki1, R. Østensen2, 3, J.-E. Solheim3, F. Johannessen3, S. L. Schuh4, J. Deetjen4 and S. Zola5

1  Dr. Remeis-Sternwarte Bamberg, Astronomisches Institut der Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
2  Isaac Newton Group of Telescopes, 37800 Santa Cruz de La Palma, Canary Islands, Spain
    e-mail: roy@ing.iac.es
3  University of Tromsø, Department of Physics, 9037 Tromsø, Norway
    e-mail: janerik@phys.uit.no
4  Institut für Astronomie und Astrophysik, Universität Tübingen, Waldhäuser Straße 64, 72076 Tübingen, Germany
    e-mail: schuh@astro.uni-tuebingen.de
5  Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Cracow, Poland
    e-mail: sfzola@cyf-kr.edu.pl

(Received 14 August 2001 / Accepted 26 September 2001 )

Abstract
We report the discovery of an eclipsing binary - HS 0705+6700 -being an sdB star with a faint companion. From its light curve the orbital period of 8263.87 s, the mass ratio of the system q = 0.28, the inclination of 84$\fdg4$ and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi-amplitude of the radial velocity curve K1 = 85.8 km s-1 and a mass function of f(m) = 0.00626 $M_{\odot}$ are determined. A spectroscopic analysis of the blue spectra results in $T_{\rm eff} = 28 800$ K, $\log g = 5.40$, and $\log (n_{\rm He}/n_{\rm H}) = -2.68$. These characteristics are typical for sdB stars, as is its mass of 0.48 $M_{\odot}$. According to its mass (0.13 $M_{\odot}$) and radius (0.19 $R_{\odot}$), the companion is an M dwarf. The primary is in a core helium burning phase of evolution, and the system must have gone through a common envelope stage when the primary was near the tip of the red giant branch.


Key words: stars: subdwarfs -- binaries: eclipsing -- binaries: spectroscopic -- stars: early-type -- stars: fundamental parameters -- stars: individual: HS 0705+6700

Offprint request: H. Drechsel, drechsel@sternwarte.uni-erlangen.de

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