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We intended to measure the radial velocity curve of the supergiant companionto the eclipsing high mass X-ray binary pulsar EXO1722-363 and hence determinethe stellar masses of the components.  We used a set of archival K$_{\rm s}$-band infrared spectra of thecounterpart to EXO1722-363 obtained using ISAAC on the VLT, andcross-correlated them in order to measure the radial velocity of the star.  The resulting radial velocity curve has a semi-amplitude of $24.5 \pm 5.0$ kms$^{-1}$. When combined with other measured parameters of the system, thisyields masses in the range 1.5 $\pm$ 0.4 - 1.6 $\pm$ 0.4 M$_{\odot}$ for theneutron star and 13.6 $\pm$ 1.6 - 15.2 $\pm$ 1.9 M$_{\odot}$ for the B0--1 Iasupergiant companion. These lower and upper limits were obtained under theassumption that the system is viewed edge-on (i = 90$^\circ$) for the lowerlimit and the supergiant fills its Roche lobe ($\beta = 1$) for the upper limitrespectively. The system inclination is constrained to $i>75^{\circ}$ and theRoche lobe-filling factor of the supergiant is $\beta>0.9$. Additionally wewere able to further constrain our distance determination to be 7.1 $\le$ d$\le$ 7.9 kpc for EXO1722-363. The X-ray luminosity for this distance range is4.7 $\times$ 10$^{35}$ $\le$ L$_{\rm X}$ $\le$ 9.2 $\times$ 10$^{36}$ ergs$^{-1}$.  EXO1722-363 therefore becomes the seventh of the ten known eclipsing X-raybinary pulsars for which a dynamical neutron star mass solution has beendetermined. Additionally EXO1722-363 is the first such system to have a neutronstar mass measurement made utilising near-infrared spectroscopy.

Preliminary determinations of the masses of the neutron star and mass donor in the High Mass X-ray Binary system EXO 1722-363