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The CCD observations of the rich open star cluster NGC 2099 and its surrounding eld region have been carried out up to a limiting magnitude of V 22 mag in B, V and I passbands for the rst time. A total of12 000 stars have been observed in the area of about 24 0 34 0 in the cluster region, as well as2180 stars in the 12 0 12 0 area of the eld region located 45 0 away from the cluster center. The cluster parameters determined by tting the convective core overshoot isochrones in the V;(B V )a ndV;(V I) diagrams are E(B V )=0 :30 0:04 mag, distance = 1360 100 pc, age = 400 Myr and metallicity Z =0 :008. A well- dened cluster main sequence spread over about 8 mag in range is observed for the rst time. Its intrinsic spread amounting to0.06 mag in colour is almost the same over the entire brightness and can be understood in terms of the presence of physical/optical binaries. The core and cluster radii determined from the radial stellar density proles are 185 00 and 1000 00 respectively. Only about 22% of cluster members are present in the core region. The eects of mass segregation, most probably due to dynamical evolution, have been observed in the cluster. The mass function slope of the entire cluster is 0:67 0:12. It becomes closer to the Salpeter value of 1:35, if flattening in the cluster mass function due to presence of both binaries and a much more extended corona is considered.

A comprehensive study of the rich open star cluster NGC 2099 based on deep $\vec{BVI}$ CCD observations