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The gamma-ray binary LS 5039: mass and orbit constraints from MOST observations

The results of a coordinated space-based photometric and ground-based spectroscopic observing campaign on the enigmatic gamma-ray binary LS 5039 are reported. Sixteen days of observations from the MOST satellite have been combined with high-resolution optical echelle spectroscopy from the 2.3m ANU Telescope in Siding Spring, Australia. These observations were used to measure the orbital parameters of the binary and to study the properties of stellar wind from the O primary. We found that any broad-band optical photometric variability at the orbital period is below the 2 mmag level, supporting the scenario that the orbital eccentricity of the system is near the 0.24 +/- 0.08 value implied by our spectroscopy, which is lower than values previously obtained by other workers. The low amplitude optical variability also implies the component masses are at the higher end of estimates based on the primary's O6.5V((f)) spectral type with a primary mass of ~26 solar masses and a mass for the compact star of at least 1.8 solar masses. The mass loss rate from the O primary was determined to be 3.7E-7 to 4.8E-7 solar masses per year.