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Fig. 2
V vs. V – I colour–magnitude diagram of NGC 6558 from NTT 2012 data. The four sample stars are shown as red stars, and RR Lyrae are shown as green filled circles. A BaSTI isochrone for an age of 13 Gyr, [Fe/H] = −1.0 and primordial helium abundance (Y = 0.25) is overplotted.
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