Post-CE evolution with time of the orbital period (P_orb), the mass transfer rate due to atmospheric Roche-lobe overflow (Ṁ_RG), RG mass (M_RG) and RG radius (R_RG). The red star indicates the present-day properties of FN Sgr, without error bars, the red rectangle the minimum and maximum mass transfer rate required to explain the WD luminosity as powered by nuclear burning, and the vertical blue line the time at which the binary has the same orbital period of FN Sgr. Our model reproduces reasonably well not only the RG mass, RG radius and the orbital period but also the mass transfer rate.