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 about 22 mag in B, V and I passbands for the first time. A total of
about 12000 stars have been observed in the area of about 24 arcmin X
34 arcmin in the cluster region, as well as about 2180 stars in the
12 arcmin X 12 arcmin area of the field region located about 45 arcmin
away from the cluster center. The cluster parameters determined by fitting the
convective core overshoot isochrones in the V, (B-V) and V, (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-defined cluster main sequence spread over about
8 mag in range is observed for the first time. Its intrinsic spread amounting to
about 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 arcsec and 1000 arcsec respectively. Only about 22 percent 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.