Post-AGB evolutionary tracks of model stars of metallicity Z = 2 × 10⁻³ (left panel) and Z = 4 × 10⁻³ (right). The green track in the right panel refers to a 0.9 M_⊙ model star, which experienced a late TP, shortly after the beginning of the contraction towards the post-AGB phase. Coloured points indicate the observationally derived position of the sources considered in the present investigation, according to the effective temperatures given in K14 and K15, and the luminosities derived in Sect. 3. Red squares and blue triangles refer to the sources surrounded by carbonaceous dust and silicates, respectively.