Spectrum RW Cephei displays many complex lines in its spectrum, many of which are stronger and more broad than usual. An initial study in 1956 focusing on the blue spectral region found many metal absorption lines with two components separated by a central maximum, attributed to emission superposed on an absorption line widened due to turbulence. The shortward absorption components were found to be significantly stronger than the longward components, caused by an outward moving shell of gas. A follow-up study in 1972 focusing on redder spectral regions found unusually strong sodium D lines too intense to be caused by the interstellar medium. The iron I line was found to be 30% stronger than in normal K-type supergiants, while the titanium I and vanadium I lines were of the same strengh or weaker. With these peculiar spectral features, the star finds no counterpart among the known hypergiants, with only Rho Cassiopeiae displaying remotely similar features. Circumstellar material The star shows evidence for a significant amount of circumstellar material in its spectrum. The IRAS low resolution spectrum shows signatures of optically thick silicate emission at 10 and 18 microns, an indication for high amounts of mass loss. Silicon monoxide emission was suspected in 1982, and later confirmed using higher resolution spectra showing clear signs of emission at 4.0, 4.04 and 4.08 microns. Direct imaging in mid-infrared bands reveals the source to be extended, having an azimuthally symmetric structure similar to IRC +10420. Spectral type The spectrum has been classified as early as G8 and as late as M2, but it isn't clear that there has been actual variation. In the first MK spectral atlas, it was listed M0:Ia. RW Cephei was later listed as the standard star for spectral type G8 Ia, then as the standard for K0 0-Ia. Based on the same spectra it was adjusted to the standard star for type K2 0-Ia. Molecular bands characteristic of M-class stars are seen in infrared spectra, but not always in optical spectra. Physical properties The temperature of RW Cep is uncertain, with contradictory excitation strengths in the spectrum. A simple colour correlation temperature fit gives temperatures around 3,749 K, while a full spectrum fit gives a temperature of 5,018 K. Another fit using J-band spectral data and MARCS stellar models gives a temperature of K. Luminosities have been derived on the basis of a membership to Cepheus OB1, with studies finding