R Andromedae has a spectral type that varies as its brightness changes. At a typical maximum it is assigned a spectral type of S5/4.5e. This makes it an
S-type star, a red giant similar to class M stars but with unusually strong molecular bands of ZrO in its spectrum compared to the
titanium oxide (TiO) bands seen in other cool giants. S stars are intermediate between
carbon stars and the more typical oxygen-rich giants. The S5 indicates its relative temperature, while the number after the slash is a measure of the relative C:O ratio, 4.5 meaning carbon is about 97% as abundant as oxygen. ZrO bands in R Andromedae are about twenty times stronger than those of TiO. When it is fainter, the spectral type has been classified as late as S8,8e. On this older classification system for S stars, the number after the comma is an indication of the relative strength of ZrO and TiO bands which used to be considered to show the C:O ratio. R Andromedae, like all Mira variables, is an
asymptotic giant branch star, one that has exhausted its core helium and is burning it in a shell outside the core and hydrogen in a shell closer to the surface. These stars undergo
dredge-up events which cause convection of fusion products to the surface and anomalies such as enhanced carbon and zirconium. Asymptotic giant branch stars are very cool and luminous red giants; R Andromedae varies in temperature and luminosity but is typically about 2,500 K and . The angular diameter of R Andromedae has been measured at and on different dates, corresponding to radii of and respectively, assuming a distance of . Other measurements based on the
spectral energy distribution of the star give a luminosity of and a very cool temperature of , which imply a very large radius of . R Andromedae is
one of the largest known stars. ==Notes==