WASP-18

WASP-18 is a magnitude 9 star located 400 ly away in the Phoenix constellation of the southern hemisphere. It has a mass of 1.29 solar masses.

The star, although similar to the Sun in terms of overall contents of heavy elements, is depleted in carbon. The carbon to oxygen molar ratio of 0.23$3.47$ for WASP-18 is well below the solar ratio of 0.55.

There is a red dwarf companion star at a separation of 3,519 AU.

Planetary system
In 2009, the SuperWASP project announced the discovery of a large, hot Jupiter type exoplanet, WASP-18b, orbiting very close to this star. It has an orbital period of less than a day and a mass 10 times that of Jupiter.

Observations from the Chandra X-ray Observatory failed to find any X-rays coming from WASP-18, and it is thought that this is caused by WASP-18b disrupting the star's magnetic field by causing a reduction in convection in the star's atmosphere. Tidal forces from the planet may also explain the higher amounts of lithium measured in earlier optical studies of WASP-18.

A 2019 study proposed a second candidate planet with a 2-day orbital period based on transit-timing variations, but a 2020 study using data from both TESS and ground-based surveys ruled out the existence of a planet with the proposed properties, setting an upper limit of 10 Earth masses on any planet with this period.