Program: GS-2008B-Q-78

The low-mass X-ray binary (LMXB) 2S 0918-549 is a candidate ultracompact system identified based on X-ray and optical spectroscopic properties. As a candidate, it has been suggested to contain a low mass, hydrogen-poor degenerate companion and have an orbital period of <60 min. In order to verify its ultracompact nature by determining the orbital period, we propose a Gemini-South GMOS observation of the source. We request 2.3 hours of telescope time, seeking to obtain a light curve of the source through time-resolved photometry. Because of strong X-ray heating of the companion star by the central neutron star in the binary, it is likely that the light curve will appear periodically modulated, revealing the orbital periodicity. If the orbital period is found, we would derive a mass-radius relation for the companion star, helping understand the physical properties of this LMXB.