We use newly observed and published near–infrared spectra , together with synthetic spectra obtained from model atmospheres , to derive physical properties of three of the latest-type T dwarfs .
A new R \approx 1700 spectrum of the T7.5 dwarf HD 3651B , together with existing data , allows a detailed comparison to the well-studied and very similar dwarf , Gl 570D .
We find that HD 3651B has both higher gravity and metallicity than Gl 570D , with best-fit atmospheric parameters of T _ { eff } = 820 - 830 K , \log g = 5.4 - 5.5 , [ m/H ] = + 0.2 and K _ { zz } = 10 ^ { 4 } cm ^ { 2 } s ^ { -1 } .
Its age is 8 – 12 Gyr and its implied mass is 60 – 70 M _ { Jupiter } .
We perform a similar analyis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data , comparing them to the well-studied T8 , 2MASS J04151954-0935066 .
We find that these two dwarfs have effectively the same T _ { eff } as the reference dwarf , and similar or slightly higher gravities , but lower metallicities .
The derived parameters are T _ { eff } = 725 - 775 K and [ m/H ] = - 0.3 ; \log g = 5.3 - 5.45 for 2MASS J09393548-2448279 and \log g = 5.0 - 5.3 for 2MASS J11145133-261823 .
The age and mass are \sim 10 Gyr and 60 M _ { Jupiter } for 2MASS J09393548-2448279 , and \sim 5 Gyr and 40 M _ { Jupiter } for 2MASS J11145133-261823 .

A serious limitation to such analyses is the incompleteness of the line lists for transitions of CH _ { 4 } and NH _ { 3 } at \lambda \leq 1.7 \mu m , which are also needed for synthesizing the spectrum of the later , cooler , Y type .
Spectra of Saturn and Jupiter , and of laboratory CH _ { 4 } and NH _ { 3 } gas , suggest that NH _ { 3 } features in the Y - and J -bands may be useful as indicators of the next spectral type , and not features in the H - and K -bands as previously thought .
However , until cooler objects are found , or the line lists improve , large uncertainties remain , as the abundance of NH _ { 3 } is likely to be significantly below the chemical equilibrium value .
Moreover inclusion of laboratory NH _ { 3 } opacities in our models predicts band shapes that are discrepant with existing data .
It is possible that the T spectral class will have to be extended to temperatures around 400 K , when water clouds condense in the atmosphere and dramatically change the spectral energy distribution of the brown dwarf .