A Class Y star is the dimmest type of star, classed as a sub-brown dwarf or planetary-mass brown dwarf that may not be large enough to undergo deuterium fusion. A typical Class Y star has a temperature of around -63 - 800 F(220 - 700 K), a mass of around 5-13 times that of Jupiter(although this is a bit vague), and a similar radius to Jupiter under most circumstances. 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: