Physical parameters and multiplicity of five southern close eclipsing binaries ^*,**

¹ Department of Optics and Quantum Electronics, University of Szeged, Hungary
² School of Physics A28, University of Sydney, NSW 2006, Australia e-mail: l.kiss@physics.usyd.edu.au
³ Department of Experimental Physics, University of Szeged, Hungary
⁴ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
⁵ Konkoly Observatory of the Hungarian Academy of Sciences, Budapest, Hungary

Received: 19 November 2006
Accepted: 2 January 2007

Abstract

Aims.We detected tertiary components of close binaries from spectroscopy and light curve modelling, investigated the light-travel time effect and the possibility of magnetic activity cycles, measured mass ratios for unstudied systems, and derived absolute parameters.

Methods.We carried out new photometric and spectroscopic observations of five bright ( 10.5 mag) close eclipsing binaries, predominantly in the southern skies. We obtained full Johnson  light curves, which were modelled with the Wilson-Devinney code. Radial velocities were measured with the cross-correlation method using IAU radial velocity standards as spectral templates. Period changes were studied with the O-C method, utilising published epochs of minimum light (XY Leo) and ASAS photometry (VZ Lib).

Results.For three objects (DX Tuc, QY Hya, V870 Ara), absolute parameters have been determined for the first time. We spectroscopically detected the tertiary components in XY Leo and VZ Lib and discovered one in QY Hya. For XY Leo we updated the light-time effect parameters and detected a secondary periodicity of about 5100 d in the O-C diagram that may hint at the existence of short-period magnetic cycles. A combination of recent photometric data shows that the orbital period of the tertiary star in VZ Lib is likely to be over 1500 d. QY Hya is a semi-detached X-ray active binary in a triple system with K and M-type components, while V870 Ara is a contact binary with the third smallest spectroscopic mass ratio for a W UMa star to date ( ± 0.030). Being close to the theoretical minimum for contact binaries, this small mass ratio suggests that V870 Ara has the potential of constraining evolutionary scenarios of binary mergers. The inferred distances to these systems are compatible with the Hipparcos parallaxes.