Spectrophotometric distances in the K band have been reported by different authors for a number of obscured Galactic H ii regions .
Almost 50 % of them show large discrepancies compared to the classical method using radial velocities measured in the radio spectral region .
In order to provide a crucial test of both methods , we selected a target which does not present particular difficulty for any method and which has been measured by as many techniques as possible .
The W3 star forming complex , located in the Perseus arm , offers a splendid opportunity for such a task .
We used the NIFS spectrograph on the Frederick C. Gillett Gemini North telescope to classify candidate “ naked photosphere ” OB stars based on 2MASS photometry .
Two of the targets are revealed to be mid O-type main sequence stars leading to a distance of d = 2.20 kpc .
This is in excellent agreement with the spectrophotometric distance derived in the optical band ( d = 2.18 kpc , ) and with a measurement of the W3 trigonometric parallax ( d = 1.95 kpc , ) .
Such results confirm that the spectrophotometric distances in the K band are reliable .
The radio derived kinematic distance , on the contrary , gives a distance twice as large ( d = 4.2 kpc , ) .
This indicates that this region of Perseus arm does not follow the Galactic rotation curve , and this may be the case also for other H ii regions for which discrepancies have been found .