A spectral classification reserved for the very coolest brown dwarfs, those with surface temperatures below about 500 K, cooler and fainter even than the T dwarfs. Such stars are so cool that water features disappear from the spectrum, as H₂O condenses into water vapour at temperatures around 150 K, the same temperature as the atmosphere of Jupiter. Temperatures of around 350 K have been determined for several Y dwarfs, and one may be as cool as 250 K.