  Ultra-compact X-ray binaries (UCXBs) are accreting systems with periods less
than 1 hour, which qualifies them to contain a degenerate donor-companion. One
would expect such systems to have the easiest theoretical explanation, compared
to other kinds of X-ray binaries. Nonetheless, current theory fails to explain
high mass transfer (MT) rates in three recently well observed long-period
UCXBs. We find that this range of MT rates can be maintained if the donor is a
remnant of an out-of-thermal-equilibrium naked core of a giant which was
revealed in a very recent episode of a common envelope (CE) event.
