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.
References
^Mora, A.; Merín, B.; Solano, E.; Montesinos, B.; De Winter, D.; Eiroa, C.; Ferlet, R.; Grady, C. A.; Davies, J. K.; Miranda, L. F.; Oudmaijer, R. D.; Palacios, J.; Quirrenbach, A.; Harris, A. W.; Rauer, H.; Collier Cameron, A.; Deeg, H. J.; Garzón, F.; Penny, A.; Schneider, J.; Tsapras, Y.; Wesselius, P. R. (2001). "EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars". Astronomy and Astrophysics. 378: 116. Bibcode:2001A&A...378..116M. doi:10.1051/0004-6361:20011098.
