EPIC 216747137: a new HW Vir eclipsing binary with a massive sdOB primary and a low-mass M-dwarf companion

TL;DR

This paper reports the discovery and detailed analysis of EPIC 216747137, a HW Vir eclipsing binary with a hot sdOB primary and a low-mass M-dwarf companion, revealing its physical parameters and orbital characteristics.

Contribution

It presents the first detailed characterization of a new HW Vir system with a massive sdOB primary and a low-mass companion, including atmospheric and orbital parameters.

Findings

Primary star Teff=40400 K, logg=5.56

Secondary star mass ~0.11 Msun

System not synchronized, primary rotation slower than orbit

Abstract

EPIC 216747137 is a new HW~Virginis system discovered by the Kepler spacecraft during its K2 "second life". Like the other HW Vir systems, EPIC 216747137 is a post-common-envelope eclipsing binary consisting of a hot subluminous star and a cool low-mass companion. The short orbital period of 3.87 hours produces a strong reflection effect from the secondary (~9% in the R band). Together with AA Dor and V1828 Aql, EPIC 216747137 belongs to a small subgroup of HW Vir systems with a hot evolved sdOB primary. We find the following atmospheric parameters for the hot component: Teff=40400$\pm$1000 K, logg=5.56$\pm$0.06, log(N(He)/N(H))=$-$2.59$\pm$0.05. The sdOB rotational velocity vsini=51$\pm$10 km/s implies that the stellar rotation is slower than the orbital revolution and the system is not synchronized. When we combine photometric and spectroscopic results with the Gaia parallax, the best solution for the system corresponds to a primary with a mass of about 0.62 Msun close to, and likely beyond, the central helium exhaustion, while the cool M-dwarf companion has a mass of about 0.11 Msun.