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Post-common-envelope binaries (PCEBs) containing a white dwarf (WD) and amain-sequence (MS) star can constrain the physics of common envelope evolutionand calibrate binary evolution models. Most PCEBs studied to date have shortorbital periods ($P_{\rm orb} \lesssim 1\,$d), implying relatively inefficientharnessing of binaries' orbital energy for envelope expulsion. Here, we presentfollow-up observations of five binaries from {\it Gaia} DR3 containingsolar-type MS stars and probable ultramassive WDs ($M\gtrsim 1.2\,M_{\odot}$)with significantly wider orbits than previously known PCEBs, $P_{\rm orb} =18-49\,$d. The WD masses are much higher than expected for systems formed viastable mass transfer at these periods, and their near-circular orbits suggestpartial tidal circularization when the WD progenitors were giants. Theseproperties strongly suggest that the binaries are PCEBs. Forming PCEBs at suchwide separations requires highly efficient envelope ejection, and we find thatthe observed periods can only be explained if a significant fraction of theenergy released when the envelope recombines goes into ejecting it. Our 1Dstellar models including recombination energy confirm prior predictions that awide range of PCEB orbital periods, extending up to months or years, canpotentially result from Roche lobe overflow of a luminous AGB star. Thisevolutionary scenario may also explain the formation of several wide WD+MSbinaries discovered via self-lensing, as well as a significant fraction ofpost-AGB binaries and barium stars.

Wide post-common envelope binaries containing ultramassive white dwarfs: evidence for efficient envelope ejection in massive AGB stars