Astronomers created R, N, & S as three additional classifications for stars with temperatures similar to K and M stars but with distinct spectral features separating them from the K or M stars. The R and N classifications have been identified as variations of carbon stars. Carbon stars differ from other cool stars in that their spectra are dominated by absorption lines dreated by molecules with carbon (e.g., CH, CN, or C2). R stars have temperatures corresponding to type K, N stars to type M. Some astronomers have started referring to either R or N class stars as a single classification - type C.
Type S stars also have temperatures similar K or M stars, but with spectra dominated by heavy oxidized molecules such as LaO, VO, or ZrO.
Astronomers have recognized two new classifications for stars with surface temperatures cooler than type M. Stars with extremely low temperature surfaces, 1300 to 2500 K, are classified as L or T stars. L and T type stars emit mostly at infrared wavelengths. These two new classifications were created in 1999, in response to the large numbers of otherwise unclassifiably cool stars which were discovered during digital all-sky surveys.
L Stars: Spectra show strong metal-hydride molecular bands (CrH & FeH), and neutral metals, but TiO and VO bands are nearly absent.
T dwarfs: Spectra show strong bands of Methane (CH4), like the spectrum of Jupiter. May be failed stars (low-mass "Brown Dwarfs").