BL Telescopii is a multiple star in the constellation Telescopium. An Algol-like eclipsing binary, the star system varies between apparent magnitudes 7.09 and 9.08 in just over 778 days (2 years 48 days), which is generally too faint to be seen with the unaided eye. This is mainly due to the system being an eclipsing binary (that is, one star passing in front of the other star and resulting in a change in brightness). The eclipse itself dims the star by two magnitudes and lasts around 104 days.
Dutch astronomer Willem Jacob Luyten noted this star to be variable in 1935. Minima were retrospectively identified in old photographic plates from 1913 and 1919, and then observed by Howarth in 1936. Initially thought to be an R Coronae Borealis variable, its true nature as an eclipsing binary became clear in the 1940s.
The primary component is a yellow supergiant, whose spectral type has been calculated as either F5Iab/b or F4Ib. It is intrinsically variable, varying in brightness by 0.02 magnitude. It has pulsations of two periods, 92.5 days and 64.8 days in length. It has been classified as a UU Herculis variable—a class of yellow supergiant with semiregular variability. These stars are thought to have affinities with Cepheid variables and lie near the instability strip on the Hertzsprung-Russell Diagram. The primary is 19.73 times the mass of the Sun (M☉), and is around 300 times its diameter, while the secondary is 7.1 M☉. The secondary was identified as an M-type star from TiO (titanium oxide) absorption bands in its spectrum.
The BL Telescopii system lies around 11000 light-years (3.3 kiloparsecs) away from the galactic plane. This, coupled with its high velocity, indicates it is a runaway star, and that some violent event catapulted it on its current trajectory; one possibility is that the secondary star was once a massive (400 M☉) star that underwent huge mass loss, such as a supernova explosion, and catapulted the system outwards.