TW Horologii is a variable star in the southern constellation of Horologium,[10] near the eastern constellation border with Reticulum. It has a ruddy hue and is dimly visible to the naked eye with an apparent visual magnitude that ranges from 5.52 down to 5.95.[3] Based on parallax measurements, it is located at a distance of approximately 1,370 light years from the Sun.[1] It is drifting further away with a radial velocity of +14 km/s.[2] In the past this star has been considered a member of the open cluster NGC 1252, but this now seems unlikely.[11]

This is an aging red giant star on the asymptotic giant branch[6] with a stellar classification of C-N4IIIb:.[4] It is one of the brightest carbon stars[10] and has a type of C2 3.5.[4] The star is classified as a semiregular variable of type SRb and has a periodicity of 158 days.[3] It has expanded to 164[1] times the radius of the Sun and, on average, is radiating 4,390 times the Sun's luminosity from its swollen photosphere at an effective temperature of 3,674 K.[1] The short-lived element technetium has been observed in the spectrum, an indicator of thermal pulses during helium shell burning.[12]

Based on the detection of excess ultraviolet excess, it is most likely a binary star system.[13] An analysis of the motion of TW Horologii suggests a low-mass companion, although the UV excess suggests it is hot.[7]

In 2013 the luminosity of Mira variables, based on Hipparcos parallaxes, was used to calibrate a Period-luminosity relationship for carbon stars. The absolute magnitude of TW Horologii was calculated to be −1.79.[2]

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