Detection of a young stellar population in the background of open clusters in the third galactic quadrant

We report the detection of a young stellar population (<= 100 Myr) in the background of nine young open clusters belonging to a homogenous sample of 30 star clusters in the third Galactic quadrant ( at 217 degrees <= l <=). Deep and accurate UBVRI photometry allows us to measure model-independent age and distance for the 260 degrees clusters and the background population with high confidence. This population is exactly the same population ( the blue plume) recently detected in three intermediate-age open clusters and suggested to be a <= 1 - 2 Gyr old population belonging to the Canis Major ( CMa) overdensity ( Bellazzini et al.; Martinez-Delgado et al.). However, we find that the young population in those three clusters and in six clusters of our sample follows the pattern of the Norma-Cygnus spiral arm as defined by CO clouds remarkably well, while in the other three program clusters it lies in the Perseus arm. We finally provide one example ( out of 21) of a cluster that does not show any background population, demonstrating that this population is not ubiquitous toward CMa.