Ultracompact X-ray binaries in globular clusters: Variability of the optical counterpart of X1832-330 in NGC 6652

Evidence is emerging that the luminous X-ray sources in the cores of globul ar clusters may often consist of, or perhaps even as a class be dominated b y, ultracompact (P less than or similar to 1 hr) binary stars. To the two s uch systems already known, in NGC 6624 and NGC 6712, we now add evidence fo r 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 avai lable observations are relatively brief, the existing Hubble Space Telescop e data indicate a strong 43.6 minute periodic modulation of the visible flu x of semiamplitude 30%. Further although the orbital period of the source i n NGC 1851 is not yet explicitly measured, we demonstrate that previous cor relations of optical luminosity with X-ray luminosity and accretion disk si ze, strengthened by recent data, strongly imply that the period of that sys tem is also less than 1 hr. Thus, currently there is evidence that four of the seven globular cluster X-ray sources with constrained periods are ultra compact, a fraction far greater than that found in X-ray binaries the field .