Stellar photometry derived from the INT/WFC Photometric H \alpha Survey of the Northern Galactic Plane ( IPHAS ) can be used to identify large , reliable samples of A0-A5 dwarfs .
For every A star , so identified , it is also possible to derive individual reddening and distance estimates , under the assumption that most selected objects are on or near the main sequence , at a mean absolute r ^ { \prime } magnitude of 1.5 – 1.6 .
This study presents the method for obtaining such samples and shows that the known reddenings and distances to the open clusters NGC 7510 and NGC 7790 are successfully recovered .
A sample of over 1000 A stars is then obtained from IPHAS data in the magnitude range 13.5 < r ^ { \prime } < 20 from the region of sky including the massive northern OB association Cyg OB2 .
Analysis of these data reveals a concentration of \sim 200 A stars over an area about a degree across , offset mainly to the south of the known 1–3 Myr old OB stars in Cyg OB2 : their dereddened r ^ { \prime } magnitudes fall in the range 11.8 to 12.5 .
These are consistent with a \sim 7 Myr old stellar population at DM = 10.8 , or with an age of \sim 5 Myr at DM = 11.2 .
The number of A stars found in this clustering alone is consistent with a lower limit to the cluster mass of \sim 10 ^ { 4 } M _ { \odot } .