2026ApJ...997L..48H

Recent Gaia astrometry has revealed thousands of main-sequence + white dwarf (MS+WD) binaries at separations of ∼0.1-10 au, including a subset hosting unusually massive (≳0.8M_⊙) WDs. We argue that s-process enrichment in the nondegenerate companion provides a powerful diagnostic for identifying WDs that formed via mergers in hierarchical triple systems. For a massive WD, standard single-star evolution requires a massive (≳4M_⊙) progenitor, yet such progenitors produce negligible s-process yields. We define IK Peg-type systems as those exhibiting this mass-yield tension-barium-enhanced companions orbiting WDs too massive to have descended from efficient s-process producers. The well-known system IK Pegasi (IK Peg) exemplifies this class. Applying this framework to published spectroscopic data reveals several additional candidates, and we estimate that a few dozen such systems should exist in the current Gaia sample. If these systems trace inner-binary mergers in primordial triples, they represent observable intermediate stages toward eventual type Ia supernovae via the double-merger pathway, as predicted by recent population synthesis models.