We report a study of the stellar content of the Near-infrared cluster [ DBS2003 ] 157 embedded in the extended H ii region GAL 331.31-00.34 , which is associated with the IRAS source 16085-5138 . JHK photometry was carried out in order to identify potential ionizing candidates , and the follow-up NIR spectroscopy allowed the spectral classification of some sources , including two O-type stars .
A combination of NIR photometry and spectroscopy data was used to obtain the distance of these two stars , with the method of spectroscopic parallax : IRS 298 ( O6 V , 3.35 \pm 0.61 kpc ) and IRS 339 ( O9 V , 3.24 \pm 0.56 kpc ) .
Adopting the average distance of 3.29 \pm 0.58 kpc and comparing the Lyman continuum luminosity of these stars with that required to account for the radio continuum flux of the H ii region , we conclude that these two stars are the ionizing sources of GAL 331.31-00.34 .
Young stellar objects ( YSOs ) were searched by using our NIR photometry and MIR data from the GLIMPSE survey .
The analysis of NIR and MIR colour-colour diagrams resulted in 47 YSO candidates .

The GLIMPSE counterpart of IRAS 16085-5138 , which presents IRAS colour indices compatible with an ultra-compact H ii region , has been identified .
The analysis of its spectral energy distribution between 2 and 100 \mu m revealed that this source shows a spectral index \alpha = 3.6 between 2 and 25 \mu m , which is typical of a YSO immersed in a protostellar envelope .
Lower limits to the bolometric luminosity and the mass of the embedded protostar have been estimated as L = 7.7 \times 10 ^ { 3 } L _ { \sun } and M = 10 M _ { \sun } , respectively , which corresponds to a B0–B1 V ZAMS star .