Mass and orbit constraints of the gamma-ray binary LS 5039
  We present the results of space-based photometric and ground-based spectroscopic observing campaigns on the gamma-ray binary LS 5039. The new orbital and physical parameters of the system are similar to former results, except we found a lower eccentricity. Our MOST-data show that any broad-band optical photometric variability at the orbital period is below the 2 mmag level. Light curve simulations support the lower value of eccentricity and imply that the mass of the compact object is higher than 1.8 solar masses. 
T. Szalai, G. E. Sarty, L. L. Kiss, J. M. Matthews, J. Vink\'o, Cs.   Kiss
astro-ph.SR