What is the coolest white dwarf known? Okay, so in this ArXiv report from 2014, scientists discovered the coolest white dwarf, with a temperature of below 3000 K. However, as this report was from over 6 years ago, I think this information may be outdated. So, as of right now, what is the coolest white dwarf, and where can I find more information about this?
The one from 2014 is still the record holder I believe - in the sense that it is reasonably convincing that the unseen companion of the pulsar PSR 2227-0137 is consistent with being a white dwarf with a surface temperature below 3000 K.
It is worth considering why such objects might be difficult to find.
(1) It is only the highest mass white dwarfs that have had time to cool below 3000 K in the lifetime of our Galaxy. The reason for this is that more massive white dwarfs are smaller and have much higher densities. This enables them to crystallise in their interiors at high temperatures and the heat capacity of a crystalline solid becomes much lower than a gas at the same temperature, meaning that they have less heat left to radiate away.  High mass white dwarfs are rare; they are the endpoints of the evolution of 6-8$M\odot$ stars, which are much less common than the 1-6$M_\odot$ stars that produce most of the lower mass white dwarfs in the galaxy.
(2) They are almost invisible. Temperatures below 3000 K are getting down into brown dwarf territory. Now we've found thousands of brown dwarfs, but these are (a) likely to be more common than massive white dwarfs (about 1 for every 5 stars) and (b) they will be about 20 times the radius of a massive white dwarf and therefore emit 400 times the flux at the same temperature. Thus it will be very difficult to find free-floating cold white dwarfs, especially if they are intrinsically rare.
(3) But can't we find them in binary systems? If the companion star is $&gt;1 M_\odot$ then it will have lived and died in the time it took the massive white dwarf to cool and the system would have to be a double-degenerate. i.e. A neutron star + white dwarf or two white dwarfs. Many such systems are known, but I think we come back to the fact that massive white dwarfs are rare, and if they are more massive than say $1.15M_\odot$ it becomes difficult to distinguish them from neutron stars if all you have is the dynamics of the system. A more promising line of attack would be to look for them as binary companions to old K and M dwarfs. But this would imply that the original binary system would have had a very high mass ratio - a fair fraction of these sytems would have become unbound as the white dwarf progenitor lost its (and most of the system's) mass, and very high mass-ratio binary systems are not that common. My guess is that the Gaia astrometric survey may be a good source of candidate cold white dwarfs because it will identify the binary motions and secondary (really, the unseen primary) masses for many, many K and M stars. Any K/M star with an unseen companion that is more massive than $1M_\odot$ will be of interest.