The Silent Majority: The Interacting Post-Common-Envelope Binaries Underlying Cataclysmic Variables

Abstract

We analyze the orbital period distribution of post-common-envelope white-dwarf-main-sequence (WDMS) binaries by cross-matching the new spectroscopic Gaia DR3 WDMS catalog with TESS light curves, and applying a uniform periodicity search and vetting pipeline. We identify 107 periodic systems, including 74 eclipsing binaries (32 new) and 33 binaries exhibiting only sinusoidal variations. Injection-recovery tests and a forward detectability model yield a completeness-corrected distribution that is well-described by a two-component function: a log-period Gaussian peaking at $P_{\mathrm{orb}}\approx 4.1$ h with $\ddot{I}ƒ\approx 1.8$ h, plus a rising component that begins near $P_{\mathrm{orb}}\approx 12.9$ h. We refer to this extended component as the long-period tail. It consists exclusively of detached non-interacting post-common-envelope binaries (PCEBs) that likely emerged from the common envelope and have not yet initiated mass transfer. In contrast, the short-period Gaussian is dominated by interacting or near-contact systems (including 22 known cataclysmic variables), consistent with high Roche-lobe filling factors. From the completeness-corrected distribution we infer that $29.8\%\pm 4.5\%$ of the spatially unresolved WDMSs in our parent catalog are close PCEBs. Binary population synthesis models with high common-envelope efficiencies overproduce long-period systems and fail to reproduce the sharp peak, whereas lower efficiencies ($\hat{I}\pm \hat{I}»\le 0.3$) match the peak more closely, yet still underpredict the tail. Our results hint at a large, currently under-classified reservoir of pre-cataclysmic variables and weakly accreting binaries, and provide new constraints on common-envelope physics.