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 } \cong 3 when B includes O , B and A supergiants ) .
tars : evolution – Stars : blue to red ratio – supergiants – Magellanic Clouds