The solar-type eclipsing binary system LL Aquarii^⋆

Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
e-mail: astro.js@keele.ac.uk

Received: 2 July 2013
Accepted: 6 August 2013

Abstract

The eclipsing binary LL Aqr consists of two late-type stars in an eccentric orbit with a period of 20.17 d. We use an extensive light curve from the SuperWASP survey augmented by published radial velocities and UBV light curves to measure the physical properties of the system. The primary star has a mass of 1.167 ± 0.009 M_⊙ and a radius of 1.305 ± 0.007 R_⊙. The secondary star is an analogue of the Sun, with a mass and radius of 1.014 ± 0.006 M_⊙ and 0.990 ± 0.008 R_⊙ respectively. The system shows no signs of stellar activity: the upper limit on spot-induced rotational modulation is 3 mmag, it is slowly rotating, has not been detected at X-ray wavelengths, and the calcium H and K lines exhibit no emission. Theoretical stellar models provide a good match to its properties for a sub-solar metal abundance of Z = 0.008 and an age of 2.5 Gyr. Most low-mass eclipsing binary systems are found to have radii larger than expected from theoretical predictions, blamed on tidally-enhanced magnetic fields in these short-period systems. The properties of LL Aqr support this scenario: it exhibits negligible tidal effects, shows no signs of magnetic activity, and matches theoretical models well.