Pre-CE evolution of the orbital period with the core mass of the WD progenitor, color-coded with the companion mass. The gray strip in the vertical direction represents the observed WD mass in KOI 3278, while the red solar symbol shows the onset of the CE evolution. When the WD progenitor becomes an evolved first giant branch star, the companion starts to accrete a non-negligible amount of mass. The WD progenitor fills its Roche lobe during the first helium shell flash, as soon as it becomes a TP-AGB star.