Illustration of the selection procedures we applied to isolate MSa stars. (a) Preliminary selection of MSa candidates on the m_F336W − m_F438W CMD (within the red lines). (b)-(c) Selection refinements using two CMDs of different colours. We show MSa stars selected from the previous panel in black and, the rest of the MS in grey. Rejected stars are those outside the two red lines. (d) Fiducial lines (in green) used to verticalise the MSa in the m_F336W − m_F438W CMD. Stars that survived the selections from panels (a)+(b)+(c) are represented in black, while other stars are in grey. (e) Verticalised ${\mathrm{\Delta }}_{m_{F336W}-m_{F438W}}^{\text{N}}$ CMD. (f)-(g) Same as panels (d) and (e) but for the m_F275W − m_F336W CMD. (h) ${\mathrm{\Delta }}_{m_{F275W}-m_{F336W}}^{\text{N}}$ versus ${\mathrm{\Delta }}_{m_{F336W}-m_{F438W}}^{\text{N}}$ TpCD of MSa stars. (e) Hess diagram of the TpCD. (f) The two defined MSa subpopulations: MSa1 (in dark yellow) and MSa2 (in light yellow).