Strömgren diagnostic diagrams obtained from deep STELLA/WiFSIP uvbyβ photometry. a) De-reddened color-color diagram (u−v) versus (b−y); b) color difference c₁ = (u−v)−(v−b) versus the blanketing difference m₁ = (v−b)−(b−y); c) Hβ index versus b−y color; d)V magnitude versus de-reddened color difference c₁. The line is the 890 Myr isochrone with solar metallicity. The light-gray dot would be a Sun in IC 4756. We note that in panels a)–c) only stars with photometric errors smaller than ≈001 are plotted, i.e., typically brighter than 17th magnitude in y.