Right: orbital period, secondary mass, and WD mass for the known PCEBs. The gray surfaces represent the maximum period of PCEBs if orbital energy is the only energy source for expelling the envelope during the CE phase. This maximum period is a function of M_sec  and M_WD. It increases with core mass for progenitors that fill their Roche lobe on the the first giant branch (i.e., for M_WD ≲ 0.47 M_⊙, gray surface in the back) and is generally long for AGB progenitors (M_WD ≳ 0.51 M_⊙, gray surface in the front). KOI-3278 (red cross) and IK Peg (red point) are the only known PCEBs with solar-type secondary stars, and both systems are located above this limit. All WD+M-dwarf PCEBs (blue points) are well below the critical value. Left: 2D projections of the right hand plot. The three black lines in the upper panel represent the maximum period for a fixed mass of the secondary star. The solid line is for 0.3 M_⊙ (typical for the WD+M-dwarf PCEBs), the dashed line for 1.042 M_⊙ (the companion in KOI-3278), and the dotted line for 1.7 M_⊙ (the companion in IK Peg).