Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995 , respectively .
A certain example of the yet cooler Y-type spectral class ( T _ { eff } \lesssim 500 K ? )
has not been seen .
Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line ( \sim 13.5 \mathrm { M } _ { \mathrm { J } } ) are rare .
Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis ( hundreds of AU ) characteristic of the lowest mass imaged companions .
As a consequence , Y-class companions with large semi-major axes should be scarce around stars < 2 Gyr old , and also around substellar primaries of all ages .
By focusing on brown dwarf companions to young stellar primaries , it is possible to derive a first estimate of the brown dwarf IMF over the entire range of brown dwarf masses ( 13 \mathrm { M } _ { \mathrm { J } } to 79 \mathrm { M } _ { \mathrm { J } } ) – the number of companion brown dwarfs is proportional to mass to the -1.2 \pm 0.2 power .