  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. 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. 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.
