  Ultracompact X-ray binaries (UCXBs) have orbital periods shorter than about
80 minutes and typically consist of a neutron star that accretes hydrogen-poor
matter from a white dwarf companion. Angular momentum loss via gravitational
wave radiation drives mass transfer via Roche-lobe overflow. The late-time
evolution of UCXBs is poorly understood -- all 13 known systems are relatively
young and it is not clear why. One question is whether old UCXBs actually still
exist, or have they become disrupted at some point? Alternatively they may be
simply too faint to see. To investigate this, we apply the theories of
dynamical instability, the magnetic propeller effect, and evaporation of the
donor, to the UCXB evolution. We find that both the propeller effect and
evaporation are promising explanations for the absence of observed long-period
UCXBs.
