We present new spectroscopic and photometric measurements of the white dwar
fs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the
coolest ones known, all with effective temperatures below 4000 K. Their me
mbership in the Galactic halo population is discussed, and detailed compari
sons of all three objects with new atmosphere models are presented. The new
models consider the effects of mixed H/He atmospheres and indicate that WD
0346+246 and F351-50 have predominantly helium atmospheres with only trace
s of hydrogen. LHS 3250 may be a double degenerate whose average radiative
temperature is between 2000 and 4000 K, but the new models fail to explain
this object.