WASP-20 is a close visual binary with a transiting hot Jupiter

TL;DR

The paper reveals that WASP-20 is a binary star system with a transiting hot Jupiter, and reanalyzes previous data to confirm the planet orbits the primary star, affecting its measured properties.

Contribution

It is the first to identify WASP-20 as a binary system and to analyze the planetary transit scenario considering both stellar components.

Findings

WASP-20 is a binary separated by 0.2578 arcseconds.

The planetary transit most likely occurs around the primary star.

Reanalysis increases the planet's mass and radius estimates.

Abstract

We announce the discovery that WASP-20 is a binary stellar system, consisting of two components separated by $0.2578\pm0.0007^{\prime\prime}$ on the sky, with a flux ratio of $0.4639\pm 0.0015$ in the $K$-band. It has previously been assumed that the system consists of a single F9 V star, with photometric and radial velocity signals consistent with a low-density transiting giant planet. With a projected separation of approximately $60$ au between the two components, the detected planetary signals almost certainly originate from the brighter of the two stars. We reanalyse previous observations allowing for two scenarios, `planet transits A' and `planet transits B', finding that both cases remain consistent with a transiting gas giant. However, we rule out the `planet transits B' scenario because the observed transit duration requires star B to be significantly evolved, and therefore have an age much greater than star A. We outline further observations which can be used to confirm this finding. Our preferred `planet transits A' scenario results in the measured mass and radius of the planet increasing by 4$\sigma$ and 1$\sigma$, respectively.