The quest for companions to post-common envelope binaries

III. A reexamination of HW Virginis

K. Beuermann¹, S. Dreizler¹, F. V. Hessman¹ and J. Deller^1,2

¹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: beuermann@astro.physik.uni-goettingen.de
² Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK

Received: 11 April 2012
Accepted: 11 June 2012

Abstract

We report new mid-eclipse times of the short-period sdB/dM binary HW Virginis, which differ substantially from the times predicted by a previous model. The proposed orbits of the two planets in that model are found to be unstable. We present a new secularly stable solution, which involves two companions orbiting HW Vir with periods of 12.7 yr and 55 ± 15 yr. For orbits coplanar with the binary, the inner companion is a giant planet with mass M₃ sin i₃ ≃ 14 M_Jup and the outer one a brown dwarf or low-mass star with a mass of M₄ sin i₄ = 30−120 M_Jup. Using the mercury6 code, we find that such a system would be stable over more than 10⁷ yr, in spite of the sizeable interaction. Our model fits the observed eclipse-time variations by the light-travel time effect alone, without invoking any additional process, and provides support for the planetary hypothesis of the eclipse-time variations in close binaries. The signature of non-Keplerian orbits may be visible in the data.