Horologium has several variable stars. R Horologii is a red giant Mira variable with one of the widest ranges in brightness known of stars in the night sky visible to the unaided eye. It is around 1,000 light-years from Earth. It has a minimum magnitude of 14.3 and a maximum magnitude of 4.7, with a period of approximately 13 months. T and U Horologii are also Mira variables. The Astronomical Society of Southern Africa reported in 2003 that observations of these two stars were needed as data on their light curves was incomplete. TW Horologii is a semiregular variable red giant star that is classified as a carbon star, and is 1,370 (±70) light-years from Earth.