  We present a photometric analysis of the star clusters Lindsay 1, Kron 3,
NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud (SMC),
observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in
the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ~3.5
mag deeper than the main-sequence turnoff points, deeper than any previous
data. Cluster ages were derived using three different isochrone models: Padova,
Teramo, and Dartmouth, which are all available in the ACS photometric system.
Fitting observed ridgelines for each cluster, we provide a homogeneous and
unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs
are best approximated by the Dartmouth isochrones for all clusters, except for
NGC419 where the Padova isochrones provided the best fit. The CMD of NGC419
shows several main-sequence turn-offs, which belong to the cluster and to the
SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC419.
Interestingly, our intermediate-age star clusters have a metallicity spread of
~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic
age-metallicity relation. We find an indication for centrally concentrated blue
straggler star candidates in NGC416, while for the other clusters these are not
present. Using the red clump magnitudes, we find that the closest cluster,
NGC419 (~50kpc), and the farthest cluster, Lindsay 38 (~67kpc), have a relative
distance of ~17kpc, which confirms the large depth of the SMC.

Astrophysics