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We report the discovery of the orbital period of the ultracompact low-massX-ray binary (LMXB) 4U 1543-624 using time-resolved optical photometry takenwith the 6.5-m Clay (Magellan II) telescope in Chile. The light curve in theSloan r' band clearly shows a periodic, sinusoidal modulation at 18.2+-0.1 minwith a fractional semiamplitude of 8%, which we identify as the binary period.This is the second shortest orbital period among all the known LMXBs, and itverifies the earlier suggestion of 4U 1543-624 as an ultracompact binary basedon X-ray spectroscopic properties. The sinusoidal shape of the opticalmodulation suggests that it arises from X-ray heating of the mass donor in arelatively low-inclination binary, although it could also be a superhumposcillation in which case the orbital period is slightly shorter. If the donoris a C-O white dwarf as previously suggested, its likely mass and radius arearound 0.03 M_sun and 0.03 R_sun, respectively. For conservative mass transferonto a neutron star and driven by gravitational radiation, this implies anX-ray luminosity of 6.5X10^36 erg/s and a source distance of 7 kpc. We alsodiscuss optical photometry of another LMXB, the candidate ultracompact binary4U 1822-000. We detected significant optical variability on a time scale ofabout 90 min, but it is not yet clear whether this was due to a periodicmodulation.Comment: 4 pages, accepted for publication in ApJ Letter

The Orbital Period of the Ultracompact Low-Mass X-Ray Binary 4U 1543-624