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)(circle dot) congruent to 0.05.e(3 Z/Zcircle dot), where Z(circle dot) = 0.02 and (B/R)(circle dot) is the value of B=R at Z(circle dot) 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)(circle dot) congruent to 3 when B includes O, B and A supergiants).