Context :

Aims : We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO 1722–363 and hence determine the stellar masses of the components .

Methods : We used a set of archival K _ { s } -band infrared spectra of the counterpart to EXO 1722–363 obtained using ISAAC on the VLT , and cross-correlated them in order to measure the radial velocity of the star .

Results : The resulting radial velocity curve has a semi-amplitude of 24.5 \pm 5.0 km s ^ { -1 } .
When combined with other measured parameters of the system , this yields masses in the range 1.5 \pm 0.4 - 1.6 \pm 0.4 M _ { \odot } for the neutron star and 13.6 \pm 1.6 - 15.2 \pm 1.9 M _ { \odot } for the B0–1 Ia supergiant companion .
These lower and upper limits were obtained under the assumption that the system is viewed edge-on ( i = 90 ^ { \circ } ) for the lower limit and the supergiant fills its Roche lobe ( \beta = 1 ) for the upper limit respectively .
The system inclination is constrained to i > 75 ^ { \circ } and the Roche lobe-filling factor of the supergiant is \beta > 0.9 .
Additionally we were able to further constrain our distance determination to be 7.1 \leq d \leq 7.9 kpc for EXO 1722–363 .
The X-ray luminosity for this distance range is 4.7 \times 10 ^ { 35 } \leq L _ { X } \leq 9.2 \times 10 ^ { 36 } erg s ^ { -1 } .

Conclusions : EXO 1722–363 therefore becomes the seventh of the ten known eclipsing X-ray binary pulsars for which a dynamical neutron star mass solution has been determined .
Additionally EXO 1722–363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy .