(a) m_F336W − m_F814W CMD of the MS stars not belonging to MSa, bMs, or rMS. We selected all stars falling between the two red diagonal lines. (b) m_F606W − m_F814W CMD of the stars selected in (a), where they clearly split into two components. The MSd stars, situated in the blue component, are preliminarily selected. (c)-(d) Selection refinements for MSd stars. (e)-(g) Fiducials (in green) used to verticalise the MSd sequence in the m_F336W − m_F438W and m_F275W − m_F336W CMDs. (f)-(h) Verticalised ${\mathrm{\Delta }}_{F336W-F438W}^{\mathrm{N}}$ and ${\mathrm{\Delta }}_{F275W-F814W}^{\mathrm{N}}$ CMDs. The TpCD (i) and Hess diagram (l) of MSd stars reveal three main subpopulations, which we define in panel (m) as MSd1 (pink), MSd2 (magenta), and MSd3 (purple).