  Ultracompact X-ray binaries (UCXBs) are thought to have relatively simple
binary evolution post-contact, leading to clear predictions of their luminosity
function. We test these predictions by studying the long-term behavior of known
UCXBs in our Galaxy, principally using data from the MAXI All-Sky Survey and
the Galactic Bulge Scans with RXTE's PCA instrument. Strong luminosity
variations are common (and well-documented) among persistent UCXBs, which
requires an explanation other than the disk instability mechanism. We measure
the luminosity function of known UCXBs in the Milky Way, which extends to lower
luminosities than some proposed theoretical luminosity functions of UCXBs. The
difference between field and globular cluster X-ray luminosity functions in
other galaxies cannot be explained by an increased fraction of UCXBs in
globular clusters. Instead, our measured luminosity function suggests that
UCXBs only make up a small fraction of the X-ray binaries above a few times
10^36 ergs/s in both old field populations and globular clusters.
