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 ZZ_, 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). tars: evolution – stars: supergiants – galaxies: Magellanic Clouds