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 : $\frac{ B/R}{ (B/R)_{ \odot}} \cong 0.05\cdot e^{ 3 \frac{ Z}{ Z_{ \odot}}}$, where $Z_{ \odot}=0.02$ and $(B/R)_{ \odot}$ is the value of $B/R$ at $Z_{ \odot}$ 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, $(B/R)_{ \odot}\cong3$ when $B$ includes O, B and A supergiants).
Categories: astro-ph