  Ultracompact X-ray binaries (UCXBs) are a distinctive but elusive family of
low-mass X-ray binaries (LMXBs) characterised by their tight orbits and
degenerate donor stars. Here we present UltraCompCAT, the first online and
comprehensive catalogue of UCXBs. The initial version of UltraCompCAT comprises
49 sources, including 20 'confirmed' UCXBs (those with a measured orbital
period shorter than 80 min) and 25 systems that we label as 'candidate' based
on their multi-wavelength phenomenology. For completeness, we also include four
LMXBs with orbital periods in the range of 80 to 120 min, since they might be
related (e.g. close progenitors) or even part of the UCXB population that
evolved towards longer periods. We discuss the orbital period and Galactic
distribution of the catalogue's sample. We provide evidence for the presence of
at least two separate groups of UCXBs. One formed by persistent systems with
orbital periods shorter than 30 min and a second group of transient objects (70
per cent) with periods in the range of 40 to 60 min. We show that the former
group is dominated by sources formed in globular clusters, while the latter
accounts for the (known) UCXB population in the Galactic field. We discuss the
possible evolutionary channels for both groups.
