R99 has a peculiar spectrum that has been described as OBf:pe,[6] "unclassifiable", peculiar WN10, "similar to the unusual LBV HD 5980",[7] "unique", and Ofpe/WN9.[8] The Ofpe/WN9 type remains even though other stars of this type have been reclassified to types between WN9 and WN11.[4] R99 has significant differences from those other stars which preclude it being given a simple WN spectral type: the ultraviolet spectrum is strongly blanketed over a different range of wavelengths; highly ionised iron lines are seen in absorption instead of emission; the Hi lines are unusually narrow and have no P Cygni profile; a lack of any significant absorption features near Hδ; a number of metal lines are unusually strong or weak compared to other stars of the type; there is a small unexplained infrared excess.[9]
The wind structure of R99 may be significantly different from most WR stars and LBVs. The normal temperature-stratified WR wind is accelerated to terminal velocity, causing lines of different ionisation levels of Helium to be created at different distances from the star. This does not seem to apply to R99.[9] Significant polarisation of the spectrum continuum is also seen, suggesting an asymmetric wind. This has not been observed in other WR stars.[10]
R99 shows brightness variations of about 0.3 magnitude over a period of decades, and smaller amplitudes with the strongest periods at two and ten days. The colour also varies, with the star being bluer at minimum light.[11] It has been classified as a luminous blue variable on account of the variability and spectrum, although it has never been observed in outburst.[3] Others still list it only as a candidate.[12][13]
^ abcdHøg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862.
^ abcDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
^Crowther, P. A.; Smith, L. J. (1997). "Fundamental parameters of Wolf-Rayet stars. VI. Large Magellanic Cloud WNL stars". Astronomy and Astrophysics. 320: 500. Bibcode:1997A&A...320..500C.
^Van Genderen, A. M.; Sterken, C.; De Groot, M.; Reijns, R. A. (1998). "Light variations of massive stars (alpha Cyg variables). XV. The LMC supergiants R99 (LBV), R103, R123 (LBV) and R128". Astronomy and Astrophysics. 332: 857. Bibcode:1998A&A...332..857V.
