  A short review of ultra-compact binaries, focused on ultra-compact X-ray
binaries, is followed by a discussion of recent results of our VLT campaign to
obtain optical spectra of (candidate) ultra-compact X-ray binaries. We find
evidence for carbon/oxygen as well as helium/nitrogen discs and no evidence for
(traces) of hydrogen. This suggests that the donors in the observed systems are
white dwarfs. However, we also find large differences between the two C/O discs
of which we have good spectra, which highlights the need for a better
understanding of the optical spectra.
