The open cluster Berkeley 90 is the home to one of the most massive binary systems in the Galaxy , LS III +46 \degr 11 , formed by two identical , very massive stars ( O3.5 If* + O3.5 If* ) , and a second early-O system ( LS III +46 \degr 12 with an O4.5 IV ( ( f ) ) component at least ) .
Stars with spectral types earlier than O4 are very scarce in the Milky Way , with no more than 20 examples .
The formation of such massive stars is still an open question today , and thus the study of the environments where the most massive stars are found can shed some light on this topic .
To this aim , we determine the properties and characterize the population of Berkeley 90 using optical , near-infrared and WISE photometry and optical spectroscopy .
This is the first determination of these parameters with accuracy .
We find a distance of 3.5 ^ { +0.5 } _ { -0.5 } kpc and a maximum age of 3 Ma .
The cluster mass is around 1 000 \ > M _ { \sun } ( perhaps reaching 1 500 \ > M _ { \sun } if the surrounding population is added ) , and we do not detect candidate runaway stars in the area .
There is a second population of young stars to the Southeast of the cluster that may have formed at the same time or slightly later , with some evidence for low-activity ongoing star formation .