Top: distribution of orbital periods that the PCEBs in our sample had just after the CE phase. Systems with C/O-core WDs (left) and He-core WDs (right) are shown in gray, while black distributions represent the entire PCEB population. Bottom: the cumulative distribution of post-CE periods of PCEBs containing He-core (gray) and C/O-core WDs (black). The corresponding KS probability is less than 5 × 10^-5, and we conclude that the two samples are not drawn from the same parent distribution.