LL Aquarii is an eclipsing binary star system in the equatorial constellation of Aquarius, abbreviated LL Aqr. At peak brightness it has a combined apparent visual magnitude of 9.23, which is too dim to be visible to the naked eye. Based on parallax measurements, it is located at a distance of approximately 447 light years from the Sun. The system is drifting closer with a heliocentric radial velocity of about −10 km/s.

In 1996, this star was found to be an Algol-type eclipsing binary based on photometric observations made with the Hipparcos space observatory. It was assigned the variable-star designation LL Aqr in 1999. This is a detached system forming a double-lined spectroscopic binary. In 2004, a more extensive light curve showed an eccentric orbit with a period of 20.1784 days. During the primary eclipse, the system dropped to magnitude 9.86, while the secondary eclipse showed a magnitude of 9.59. The first orbital elements were published in 2008, showing an orbital eccentricity of 0.3095 with a mass ratio of 0.86. Stellar models indicated the stars are near the mid point of their main sequence lifetimes.

The more massive member of the system, the primary component, has a stellar classification of F9 V, matching an F-type main-sequence star. It has 19.5% more mass than the Sun and a 32% greater girth. This star is radiating 2.15 times the luminosity of the Sun from its photosphere at an effective temperature of around 6,080 K. It is spinning with a projected rotational velocity of 3.5±0.5 km/s. The metallicity, or abundance of elements with mass greater than helium, is very nearly Sun-like.

The secondary component is considered a solar twin, which means its properties are close to Sun-like. It is a G-type main-sequence star with a class of G3 V. Neither member of the system shows signs of stellar activity, being slowly rotating and not emitting X-rays. Tidal effects between the two stars is negligible; they have an orbital separation of 40.7 times the radius of the Sun.