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Aims: A near-infrared study of the main ionizing star of the ultracompact HIIregion G23.96+0.15 (IRAS 18317-0757) is presented, along with a re-evaluationof the distance to this source, and a re-assessment of H- and K-bandclassification diagnostics for O dwarfs; Methods: We have obtained near-IRVLT/ISAAC imaging and spectroscopy of G23.96+0.15, plus archival imaging fromUKIRT/UFTI. A spectroscopic analysis was carried out using a non-LTE modelatmosphere code; Results: A quantitative H- and K-band classification schemefor O dwarfs is provided, from which we establish an O7.5V spectral subtype forthe central star of G23.96+0.15. We estimate an effective temperature of Teff ~38 kK from a spectral analysis; Conclusions: A spectroscopic distance of 2.5kpc is obtained for G23.96+0.15, substantially lower than the kinematicdistance of 4.7 kpc, in common with recent studies of other Milky way HIIregions. Such discrepancies would be alleviated if sources are unresolvedbinaries or clusters.

On the central ionizing star of G23.96+0.15 and near-IR spectral classification of O stars