Intermediate-Separation White-Dwarf Binaries in the Gaia Era

The Gaia mission has unveiled a previously unexplored region in the parameter space of binary systems hosting white-dwarf companions. These systems, with orbital separations around 1AU and often non-zero eccentricities, are unexpected given the interactions during the white-dwarf progenitor's asymptotic giant branch (AGB) phase. Consequently, they challenge our current understanding of mass transfer, and are not represented in standard binary population synthesis studies. Moreover, they appear to be related to other post-AGB stars, such as barium stars, CH stars, carbon-enhanced metal-poor stars, blue stragglers/lurkers, symbiotic stars, and hot subdwarfs. In this talk, I will present the distinctive properties of these systems, as revealed by an unprecedented sample that we have recently identified in Gaia DR3 data, and discuss their implications for binary evolution. I will also outline our roadmap for follow-up observations and detailed characterization in anticipation of Gaia DR4.