The blue to red supergiant ratio in young clusters at various metallicities

We present new determinations of the blue to red supergiant ratio (B/R) in young open clusters at various metallicities. For this purpose, we examine the HR diagrams of 45 clusters in the Galaxy and of 4 clusters in the Magellanic Clouds. The identification of supergiants is based on spectroscopic measurements (with photometric counts to check the results). The new counts confirm the increase of the B/R ratio when the metallicity increases with the following normalized relation : B/R (B/R)⊙ = 0.05 ·e 3 Z Z⊙ , where Z⊙ = 0.02 and (B/R)⊙ is the value of B/R at Z⊙ which depends on the definition of B and R and on the age interval considered (e.g. for spectroscopic counts including clusters with log age between 6.8 and 7.5, (B/R)⊙ = 3 when B includes O, B and A supergiants).