IK Pegasi and the Double-merger Path to Type Ia Supernovae

Abstract 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 (≳0.8 M ⊙ ) 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 (≳4 M ⊙ ) 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.