R Monocerotis is a T Tauri variable

T Tauri star

T Tauri stars are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.-Characteristics:...

 in the constellation Monoceros

Monoceros

Monoceros is a faint constellation on the celestial equator. Its name is Greek for unicorn. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south and Hydra to the east...

. The apparent magnitude

Apparent magnitude

The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of R Mon varies between 10 and 12.

The star is located in a diffuse nebula called "Hubble's Variable Nebula" (NGC 2261). According to some astronomers, the "star" R Monocerotis is not more than a very bright concentration of gases within the nebula, and some sky catalogues and atlases do not list it as a star.