A Class Y star is the dimmest type of star, classed as a sub-brown dwarf that may not be large enough to undergo deuterium fusion. A typical Class Y star has a temperature of around 300 - 400 K, a mass of around 7-13 times that of Jupiter, and a similar radius to Jupiter. They are often classified as large rogue planets.
WISE 0855−0714, the coldest known brown dwarf
These are the 7 Y-type stars discovered by J. Davy Kirkpatrick et al. in 2012: