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The maximum mass of a neutron star (NS) is poorly defined. Theoreticalattempts to define this mass have thus far been unsuccessful. Observationalresults currently provide the only means of narrowing this mass range down.Eclipsing X-ray binary (XRB) pulsar systems are the only interacting binariesin which the mass of the NS may be measured directly. Only 10 such systems areknown to exist, 6 of which have yielded NS masses in the range 1.06 - 1.86M$_{\odot}$.We present the first orbital solutions of two further eclipsingsystems, OAO 1657-415 and EXO 1722-363, whose donor stars have only recentlybeen identified. Using observations obtained using the VLT/ISAAC NIRspectrograph, our initial work was concerned with providing an accuratespectral classification of the two counterpart stars, leading to a consistentexplanation of the mechanism for spin period evolution of OAO 1657-415.Calculating radial velocities allowed orbital solutions for both systems to becomputed. These are the first accurate determinations of the NS and counterpartmasses in XRB pulsar systems to be made employing NIR spectroscopy.

High Mass X-ray Binaries in the NIR : Orbital solutions of two highly obscured systems