  A number of so-called ultra-cool white dwarfs have been detected in different
surveys so far. However, based on anecdotal evidence it is believed that most
or all of these ultra-cool white dwarfs are low-mass products of binary
evolution and thus not representative for the oldest white dwarfs. Their low
mass causes relatively high luminosity making them the first cool white dwarfs
detected in relatively shallow surveys. Deeper observations are needed for the
oldest, high mass white dwarfs with the longest cooling times. We report
results of an ongoing project that combines deep IR and optical data. This
combination plus proper motion information will allow an unambiguous
identification of very cool white dwarfs, since the spectral energy
distributions are very different from other types of stellar objects. The
atmospheric parameters that can be derived from the spectral energy
distributions together with the proper motions inferred from the IR data can be
used to construct the white dwarf luminosity functions for the thick disc and
halo populations. From these we will be able to test the early star formation
history and initial mass function of the first stellar populations.
