2025ApJ...995..180G

We present a detailed analysis of the early post-mass-transfer binary system HD 698 (V742 Cas), combining high-resolution optical spectroscopy, long-baseline interferometry, and radiative transfer modeling. Counter-phased RV curves reveal a circular orbit with a period of 55.927 ± 0.001 d and component masses of M_Be = 7.48 ± 0.07 M_⊙ and M_comp = 1.23 ± 0.02 M_⊙. The Be primary is traced via broad Hα wings, while narrow metallic absorption lines originate from a slowly rotating companion. The angular separation measured via interferometry implies a dynamical distance of 888 ± 5 pc. The spectral energy distribution is best reproduced with a color excess E(B - V) = 0.321 ± 0.016 due to interstellar reddening and a moderately dense viscous decretion disk with base density ρ₀ ≃ 5 × 10^–12 g cm^–3 at r = R_eq, declining radially as ρ(r) ∝ r^-n^ with n = 3.0. The companion is found to be a luminous and inflated star with ${T}_{{\rm{eff}},{\rm{comp}}}=10.{0}_{-0.1}^{+0.2}$ kK, ${R}_{\mathrm{comp}}=13.{1}_{-0.2}^{+0.2}\,{R}_{\odot }$, and $\mathrm{log}L/{L}_{\odot }=3.19$, contributing significantly to the flux (L_comp/L_Be ∼ 0.3). Spectral line mismatches provide further circumstantial evidence that the companion is hydrogen poor, consistent with a stripped-envelope star enriched by CNO processing. HD 698 thus belongs to the emerging class of Be + bloated O/B binaries, representing a short-lived, high-luminosity post-mass-transfer phase, when the stripped donor is still spectroscopically detectable before reaching the subdwarf phase.