2026ApJ...997L..24C

We report the discovery of LAMOST J0041+3948, the most luminous post-asymptotic giant-branch (AGB) Type II Cepheid known, located in the Andromeda Giant Stellar Stream. Its spectral energy distribution (SED) exhibits a strong near-infrared excess, indicating the presence of a circumbinary dusty disk and hence binarity. SED fitting yields an effective temperature of ${T}_{{\rm{eff}}}=673{8}_{-262}^{+234}$ K and a post-AGB luminosity of $\mathrm{log}(L/{L}_{\odot })=4.3{2}_{-0.08}^{+0.07}$. Comparison with theoretical evolutionary tracks suggests a ∼2.0-4.0 M_⊙ progenitor when accounting for a possible scattered-light contribution. Zwicky Transient Facility light curves reveal a pulsation period of 89 days that lies close to the period-luminosity relation for long-period RV Tauri stars. Follow-up spectroscopy reveals clear s-process enrichment and signatures consistent with an accretion disk around the companion. The inferred progenitor is significantly younger and more massive than a typical stream member, suggesting that an additional mechanism, such as a stellar merger, is required. We propose a formation channel in which the present post-AGB binary descends from a hierarchical triple system. In this scenario, the inner binary merged after the system was displaced to its current location by the galaxy merger event, and the resulting massive merger remnant subsequently evolved into the extremely luminous post-AGB star observed today.