Formation channel for the main P − q branch of wide-period composite sdB binaries. Initially, a 0.7 M_⊙–2.0 M_⊙ main-sequence primary (MS) evolves to become a developed red giant (RG), which initiates mass transfer through Roche lobe overflow. The mass transfer rate gradually grows and exceeds 10⁻⁵ M_⊙ yr⁻¹, at which point the accretor cannot accept the incoming mass and ejects it in an outflow (shown schematically). Even though the red giant loses all its envelope, its degenerate core ignites helium and turns into a luminous sdB star. The companion main-sequence star accretes only a small amount of mass of ≲0.05 M_⊙, but, nevertheless, becomes polluted (MS^*). The radius of the RG is strongly sensitive to the metallicity of the primary, which in turn correlates with its age and mass due to Galactic evolution. This way, the Galactic age-metallicity correlation is an important factor determining the final periods of long-period sdB binaries.