Using Keck laser guide star adaptive optics imaging , we have found that the T9 dwarf WISE J1217+1626 and T8 dwarf WISE J1711+3500 are exceptional binaries , with unusually wide separations ( \approx 0.8″ , 8–15 AU ) , large near-IR flux ratios ( \approx 2–3 mags ) , and small mass ratios ( \approx 0.5 ) compared to previously known field ultracool binaries .
Keck/NIRSPEC H -band spectra give a spectral type of Y0 for WISE J1217+1626B , and photometric estimates suggest T9.5 for WISE J1711+3500B .
The WISE J1217+1626AB system is very similar to the T9+Y0 binary CFBDSIR J1458+1013AB ; these two systems are the coldest known substellar multiples , having secondary components of \approx 400 K and being planetary-mass binaries if their ages are \lesssim 1 Gyr .
Both WISE J1217+1626B and CFBDSIR J1458+1013B have strikingly blue Y - J colors compared to previously known T dwarfs , including their T9 primaries .
Combining all available data , we find that Y - J color drops precipitously between the very latest T dwarfs and the Y dwarfs .
The fact that this is seen in ( coeval , mono-metallicity ) binaries demonstrates that the color drop arises from a change in temperature , not surface gravity or metallicity variations among the field population .
Thus , the T/Y transition established by near-IR spectra coincides with a significant change in the \approx 1 µm fluxes of ultracool photospheres .
One explanation is the depletion of potassium , whose broad absorption wings dominate the far-red optical spectra of T dwarfs .
This large color change suggests that far-red data may be valuable for classifying objects of \lesssim 500 K .