Orbital period-luminosity plot of our binary post-RGB models with initial primary masses below 2.0 M_⊙, indicated by markers and colour-coded by metallicity. Also shown are the luminosity–orbital period relations derived from single-star donor models, colour-coded by metallicity and initial mass. Solid and dashed lines show those relations with and without atmospheric RLOF, respectively. The observed luminosity range of post-RGB stars is denoted by the non-darkened area (50 <  L/L_⊙ <  2500). The bottom panel shows the relative difference between the estimated orbital period from the single-star models, $P_{\mathrm{orb}}^{\mathrm{single}}$, and the orbital period from the binary models, $P_{\mathrm{orb}}^{\mathrm{binary}}$, for the same luminosity, colour-coded by metallicity and initial mass of the single-star model.