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

We report the discovery of the orbital period of the ultracompact low-mass X-ray binary (LMXB) 4U 1543-624 using time-resolved optical photometry taken with the 6.5-m Clay (Magellan II) telescope in Chile. The light curve in the Sloan r' band clearly shows a periodic, sinusoidal modulation at 18.2+-0.1 min with 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 it verifies the earlier suggestion of 4U 1543-624 as an ultracompact binary based on X-ray spectroscopic properties. The sinusoidal shape of the optical modulation suggests that it arises from X-ray heating of the mass donor in a relatively low-inclination binary, although it could also be a superhump oscillation in which case the orbital period is slightly shorter. If the donor is a C-O white dwarf as previously suggested, its likely mass and radius are around 0.03 M_sun and 0.03 R_sun, respectively. For conservative mass transfer onto a neutron star and driven by gravitational radiation, this implies an X-ray luminosity of 6.5X10^36 erg/s and a source distance of 7 kpc. We also discuss optical photometry of another LMXB, the candidate ultracompact binary 4U 1822-000. We detected significant optical variability on a time scale of about 90 min, but it is not yet clear whether this was due to a periodic modulation.