Physical parameters and multiplicity of five southern close eclipsing binaries

Abstract

Aims: Detect tertiary components of close binaries from spectroscopy and light curve modelling; investigate light-travel time effect and the possibility of magnetic activity cycles; measure mass-ratios for unstudied systems and derive absolute parameters. Methods: We carried out new photometric and spectroscopic observations of five bright (V<10.5 mag) close eclipsing binaries, predominantly in the southern skies. We obtained full Johnson BV 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 detect spectroscopically the tertiary components in XY Leo, VZ Lib and discover one in QY Hya. For XY Leo we update the light-time effect parameters and detect a secondary periodicity of about 5100 d in the O$-$C diagram that may hint about 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 (q=0.082+/-0.030). This small mass-ratio, being close to the theoretical minimum for contact binaries, 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.