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 ^ { \prime } band clearly shows a periodic , sinusoidal modulation at 18.2 \pm 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 1.4 M _ { \sun } neutron star and driven by gravitational radiation , this implies an X-ray luminosity of 6.5 \times 10 ^ { 36 } erg s ^ { -1 } and a source distance of \approx 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 .