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We report near-infrared observations of the supergiant donor to the eclipsinghigh mass X-ray binary pulsar IGR J18027-2016. We aim to determine its spectraltype and measure its radial velocity curve and hence determine the stellarmasses of the components. ESO/VLT observations of the donor utilising the NIRspectrograph ISAAC were obtained in the H and K bands. The multi-epoch H bandspectra were cross-correlated with RV templates in order to determine a radialsolution for the system. The spectral type of the donor was confirmed as B0-1I. The radial velocity curve constructed has a semi-amplitude of $23.8 \pm 3.1$km s$^{-1}$. Combined with other measured system parameters, a dynamicallydetermined neutron star mass of 1.4 $\pm$ 0.2 - 1.6 $\pm$ 0.3 M$_{\odot}$ isfound. The mass range of the B0-B1 I donor was 18.6 $\pm$ 0.8 - 21.8 $\pm$ 2.4~M$_{\odot}$. These lower and upper limits were obtained under the assumptionthat the system is viewed edge-on (i = 90$^\circ$ with $\beta$ = 0.89) for thelower limit and the donor fills its Roche lobe ($\beta = 1$ with i =73.1$^\circ$) for the upper limit respectively.

The masses of the neutron and donor star in the high mass X-ray binary IGR J18027-2016