KIC 3868420: A High-amplitude <i>δ</i> Scuti– <i>γ</i> Dor Hybrid Star Crossing the Hertzsprung Gap

Abstract We report a photometric and asteroseismic analysis of KIC 3868420, a newly identified high-amplitude δ Scuti– γ Doradus hybrid star located in the Hertzsprung gap—a short-lived and rarely observed post-main-sequence phase. Using 4 yr of Kepler long-cadence photometry, we detect 36 significant frequencies, including 11 independent modes spanning both low- and high-frequency regimes. Grid-based modeling with MESA and GYRE, including rotation, shows that five independent frequencies match a combination of radial p -modes and nonradial g -modes, supporting its hybrid nature. The best-fit models yield an evolved post-main-sequence star ( M ∼ 2.26–2.30 M ⊙ , R ∼ 4.41–4.43 R ⊙ , and τ ∼ 5.4 × 10 8 yr), although degeneracies from rotation and unknown inclination preclude a unique solution. KIC 3868420 thus represents a rare example of a high-amplitude hybrid pulsator in the Hertzsprung gap, demonstrating that high-amplitude pulsation can coexist with mixed p -/ g -mode behavior in a rapidly evolving star. This finding highlights the value of space-based photometry for identifying such rare objects.