The Likely Orbital Period of the Ultracompact Low-mass X-ray Binary 2S 0918-549

We report the discovery of the likely orbital period of the ultracompact low-mass X-ray binary (LMXB) 2S 0918-549. Using time-resolved optical photometry carried out with the 8 m Gemini South Telescope, we obtained a 2.4 hr long, Sloan r' light curve of 2S 0918-549 and found a periodic, sinusoidal modulation at 17.4 ± 0.1 minutes with a semiamplitude of 0.015 ± 0.002 mag, which we identify as the binary period. In addition to 4U 0513-40 in the globular cluster NGC 1851 and the Galactic disk source 4U 1543-624, 2S 0918-549 is the third member of the ultracompact LMXBs that have orbital periods around 18 minutes. Our result verifies the suggestion that 2S 0918-549 is an ultracompact binary based on its X-ray and optical spectroscopic properties. Given that the donor in 2S 0918-549 has been suggested to be either a C-O or He white dwarf, its likely mass and radius are around 0.024-0.029 M _sun and 0.03-0.032 R _sun, respectively, for the former case and 0.034-0.039 M _sun and 0.033-0.035 R _sun for the latter case. If the optical modulation arises from X-ray heating of the mass donor, its sinusoidal shape suggests that the binary has a low inclination angle, probably around 10°.