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The eclipsing binary LL Aqr consists of two late-type stars in an eccentricorbit with a period of 20.17 d. We use an extensive light curve from theSuperWASP survey augmented by published radial velocities and UBV light curvesto measure the physical properties of the system. The primary star has a massof 1.167 +/- 0.009 Msun and a radius of 1.305 +/- 0.007 Rsun. The secondarystar is an analogue of the Sun, with a mass and radius of 1.014 +/- 0.006 Msunand 0.990 +/- 0.008 Rsun respectively. The system shows no signs of stellaractivity: the upper limit on spot-induced rotational modulation is 3 mmag, itis slowly rotating, has not been detected at X-ray wavelengths, and the calciumH and K lines exhibit no emission. Theoretical stellar models provide a goodmatch to its properties for a sub-solar metal abundance of Z = 0.008 and an ageof 2.5 Gyr. Most low-mass eclipsing binary systems are found to have radiilarger than expected from theoretical predictions, blamed on tidally-enhancedmagnetic fields in these short-period systems. The properties of LL Aqr supportthis scenario: it exhibits negligible tidal effects, shows no signs of magneticactivity, and matches theoretical models well.

The solar-type eclipsing binary system LL Aquarii