L stars are cool stars (1500K-2000K) characterized by the
presence of molecular bands of
metal hydrides (vs. M star metal oxides) FeH, CaH,
CrH, and VO. No TiO or ZrO is present. Very cool, very light.
Also present may be Li, Cs, Rb, K, and Na. Methane (CH4) ?
A new (1998) spectral designation put forth by the 2MASS gang!
Check out these
2MASS papers on Low-Mass Stars and Brown Dwarfs