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We report near‐infrared radial velocity (RV) measurements of the recently identified donor star in the high mass X‐ray binary (HMXB) system OAO 1657−415 obtained in the  H  band using ISAAC on the Very Large Telescope.  Cross‐correlation methods were employed to construct a RV curve with a semi‐amplitude of 22.1 ± 3.5 km s −1 . Combined with other measured parameters of this system it provides a dynamically determined neutron star (NS) mass of 1.42 ± 0.26 M ⊙  and a mass of 14.3 ± 0.8 M ⊙  for the Ofpe/WN9 highly evolved donor star.  OAO 1657−415 is an eclipsing HMXB pulsar with the largest eccentricity and orbital period of any within its class. Of the 10 known eclipsing X‐ray binary pulsars OAO 1657−415 becomes the ninth with a dynamically determined NS mass solution and only the second in an eccentric system. Furthermore, the donor star in OAO 1657−415 is much more highly evolved than the majority of the supergiant donors in other HMXBs, joining a small but growing list of HMXBs donors with extensive hydrogen depleted atmospheres.  Considering the evolutionary development of OAO 1657−415, we have estimated the binding energy of the envelope of the mass donor and find that there is insufficient energy for the removal of the donor’s envelope via spiral‐in, ruling out a common envelope evolutionary scenario. With its non‐zero eccentricity and relatively large orbital period the identification of a definitive evolutionary pathway for OAO 1657−415 remains problematic, we conclude by proposing two scenarios which may account for OAO 1657−415 current orbital configuration.

The evolution and masses of the neutron star and donor star in the high mass X‐ray binary OAO 1657−415 ★