The CCD observations of the rich open star cluster NGC 2099 and its surrounding field region have been carried out up to a limiting magnitude of V ∼ 22 mag in B, V and I passbands for the first time. A total of ∼12 000 stars have been observed in the area of about 24 × 34 in the cluster region, as well as ∼2180 stars in the ∼12 × 12 area of the field region located ∼45 away from the cluster center. The cluster parameters determined by fitting the convective core overshoot isochrones in the V,

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... 4 mag, distance = 1360 ± 100 pc, age = 400 Myr and metallicity Z = 0.008. A welldefined cluster main sequence spread over about 8 mag in range is observed for the first time. Its intrinsic spread amounting to ∼0.06 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 profiles are 185 and 1000 respectively. Only about 22% of cluster members are present in the core region. The effects 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.