  Evidence is emerging that the luminous X-ray sources in the cores of globular
clusters may often consist of, or perhaps even as a class be dominated by,
ultracompact (P < 1 hr) binary stars. To the two such systems already known, in
NGC 6624 and NGC 6712, we now add evidence for two more. We detect large
amplitude variability in the candidate optical counterpart for the X-ray source
in the core of NGC 6652. Although the available observations are relatively
brief, the existing Hubble Space Telescope data indicate a strong 43.6 min
periodic modulation of the visible flux of semi-amplitude 30%. Further,
although the orbital period of the source in NGC 1851 is not yet explicitly
measured, we demonstrate that previous correlations of optical luminosity with
X-ray luminosity and accretion disk size, strengthened by recent data, strongly
imply that the period of that system is also less than 1 hr. Thus currently
there is evidence that 4 of the 7 globular cluster X-ray sources with
constrained periods are ultracompact, a fraction far greater than that found in
X-ray binaries the field.

Astrophysics