Context :

Aims : We report near-infrared observations of the supergiant donor to the eclipsing high mass X-ray binary pulsar IGR J18027-2016 .
We aim to determine its spectral type and measure its radial velocity curve and hence determine the stellar masses of the components .

Methods : ESO/VLT observations of the donor utilising the NIR spectrograph ISAAC were obtained in the H and K bands .
The multi-epoch H band spectra were cross-correlated with RV templates in order to determine a radial solution for the system .

Results : The spectral type of the donor was confirmed as B0-1 I .
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 dynamically determined neutron star mass of 1.4 \pm 0.2 - 1.6 \pm 0.3 M _ { \odot } is found .
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 assumption that the system is viewed edge-on ( i = 90 ^ { \circ } with \beta = 0.89 ) for the lower limit and the donor fills its Roche lobe ( \beta = 1 with i = 73.1 ^ { \circ } ) for the upper limit respectively .

Conclusions :