Str\"omgren metallicities for intermediate-age and old star clusters
  We report results that show that the straightforwardly star cluster
metallicities obtained from Str\"omgren $vby$ photometry is age-dependent and
need to be corrected for further use. This outcome arises from the comparison
of [Fe/H] values derived from Str\"omgren photometry with those metallicities
published in the literature for 26 Large and Small Magellanic Cloud star
clusters, whose ages range from $\sim$ 1 Gyr up to the known oldest globular
clusters' ages in these galaxies. While deriving mean star cluster
metallicities we carried out a thorough selection of red giant branch
candidates to comply with the Str\"omgren metallicity calibration validity
regime. We paid attention to the effect of contamination by field stars,
particularly of those that lie inside the star clusters' radii, distributed
along the star cluster red giant branches, and with [Fe/H] values covering a
similar range as that for the selected stars. We found that the measured
Str\"omgren metallicities are systematically more metal-poor than the published
ones and that a quadratically age-varying function reproduces the relative
metallicity values with an overall uncertainty of $\sim$ 0.05 dex. We finally
performed a similar comparison relying on a fully independent approach, that
consisted in using theoretical red giant branches of old globular clusters
spanning [Fe/H] values from -2.0 up to 0.0 dex as standard ones. We then
superimposed on to them the red giant branches of star clusters with ages in
the range 1.0 - 12.5 Gyr and estimated by interpolation their associated
metallicities. The derived theoretical relative metallicities follow a similar
trend as a function of the star clusters' ages than that found from
observations of star clusters.
