  We present the results of our investigation of the intermediate-age star
cluster BS 90, located in the vicinity of the HII region N 66 in the SMC,
observed with HST/ACS. The high-resolution data provide a unique opportunity
for a very detailed photometric study performed on one of the rare
intermediate-age rich SMC clusters. The complete set of observations is
centered on the association NGC 346 and contains almost 100,000 stars down to V
~28 mag. In this study we focus on the northern part of the region, which
covers almost the whole stellar content of BS 90. We construct its stellar
surface density profile and derive structural parameters. Isochrone fits on the
CMD of the cluster results in an age of about 4.5 Gyr. The luminosity function
is constructed and the present-day mass function of BS 90 has been obtained
using the mass-luminosity relation, derived from the isochrone models. We found
a slope between -1.30 and -0.95, comparable or somewhat shallower than a
typical Salpeter IMF. Examination of the radial dependence of the mass function
shows a steeper slope at larger radial distances, indicating mass segregation
in the cluster. The derived half-mass relaxation time of 0.95 Gyr suggests that
the cluster is mass segregated due to its dynamical evolution. From the
isochrone model fits we derive a metallicity for BS 90 of [Fe/H]=-0.72, which
adds an important point to the age-metallicity relation of the SMC. We discuss
our findings on this relation in comparison to other SMC clusters.
