  We report near-infrared radial velocity measurements of the recently
identified donor star in the high mass X-ray binary system OAO 1657-415
obtained in the H band using ISAAC on the VLT. Cross-correlation methods were
employed to construct a radial velocity curve with a semi-amplitude of $22.1
\pm 3.5$ km s$^{-1}$. Combined with other measured parameters of this system
this provides a dynamically determined neutron star mass of $1.42 \pm 0.26
M$_{\odot}$ and a mass of $14.3 \pm 0.8 M$_{\odot}$ for the Ofpe/WN9 highly
evolved donor star. OAO 1657-415 is an eclipsing High Mass X-ray binary pulsar
with the largest eccentricity and orbital period of any within its class. Of
the ten known eclipsing X-ray binary pulsars OAO 1657-415 becomes the ninth
with a dynamically determined neutron star 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 High Mass
X-ray binaries (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
OAO1657-415 remains problematic, we conclude by proposing two scenarios which
may account for OAO1657-415 current orbital configuration.
