Trumpler 5 RRL (star) and cluster members (light gray) shown (from left to right) in the plane of the sky (Dec versus RA), proper motions (pmDec versus pmRA), parallax, and Gaia CMD. In the first panel, r_t and r₅₀ are shown with the dotted and dashed lines, respectively corresponding to the tidal radius and radius enclosing 50% of the members according to Hunt & Reffert (2023). In the fourth panel, the dotted lines correspond to instability strip (IS) limits from Marconi et al. (2015) for the cluster’s metallicity; the dashed line shows the expected apparent G-band magnitude for the HB (from the empirical PLZ relation) at the cluster’s metallicity ([Fe/H] = −0.4, Donati et al. 2015), and the star’s parallax distance and extinction according to Green et al. (2019). The shading represents the uncertainty in the HB luminosity corresponding to the full range of (observed) RRL metallicities [Fe/H] ∈ [0., 2.0] dex and the uncertainties in the apparent magnitude and colour of the RRL are smaller than the star symbol. The agreement with the RRL’s apparent magnitude is remarkable and the star lies right at the edge of the limits of the IS (see the discussion in Sect. 4.2.1).