Abstract
Abstract 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 H α 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 ρ 0 ≃ 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 eff , comp = 10 . 0 − 0.1 + 0.2 kK, R comp = 13 . 1 − 0.2 + 0.2 R ⊙ , and log L / L ⊙ = 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.