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We report the detection of a young stellar population ($\leq$100 Myrs) in theba ckground of 9 young open clusters belonging to a homogenoeous sample of 30star clusters in the Third Galactic Quadrant (at $217^o \leq l \leq 260^o$).Deep and accurate UBVRI photometry allows us to measure model-independent ageand distance for the clusters and the background population with highconfidence. This population is exactly the same population (the Blue Plume)recently detected in 3 intermediate-age open clusters and suggested to be a$\leq$ 1-2 Gyr old population belonging to the Canis Major (CMa) over-density(Bellazzini et al. 2004, Mart\'inez-Delgado et al. 2005). However, we find thatthe young population in those three and in six clusters of our sample followsremarkably well the pattern of the Norma-Cygnus spiral arm as defined by COclouds, while in the other three program clusters it lies in the Perseus arm.We finally provide one example (out of 21) of a cluster which does not show anybackground population, demonstrating that this population is not ubiquitoustoward CMa.Comment: 13 pages, 3 eps figure, the first one HUGELY reduced in resolution,  accepted for publication as ApJ Lette

Detection of a Young Stellar Population in the Background of Open Clusters in the Third Galactic Quadrant