We report the detection of a young stellar population (< 100 Myrs) in the background
of nine young open clusters in the Third Galactic Quadrant. Deep and accurate UBVRI photometry allows
us to measure model-independent age and distance for the clusters and the background population
with high confidence. This population is exactly the same population (the Blue Plume)
recently detected in 3 intermediate-age open clusters and suggested to be
a < 1-2 Gyr old population belonging to the Canis Major (CM) over-density (Bellazzini et al. 2004,
Martínez-Delgado et al. 2005).
However, we find that the young population in those three and in six clusters of our sample
follows remarkably well the pattern of the Norma-Cygnus spiral arm as defined by CO clouds,
while in the other three program clusters it lies in the Perseus arm.
We finally provide examples of open clusters which do not show any background population,
demonstrating that this population is not ubiquitous toward CM.