2025ApJ...981L..35E

Recently, a double-lined binary classical Cepheid, OGLE-LMC-CEP-1347, was discovered with the orbital period (P_orb = 59 days), 5 times shorter than that of any binary Cepheid known before. The expected mass of the Cepheid was below 3.5 M_⊙, which, if confirmed, would also probe uncharted territory. The system configuration also pointed to the Cepheid being a merger. We present a novel method for determining precise physical parameters of binary Cepheids using both theory and observations. This q-PED method combines the measured mass ratio (q), pulsation (P), and evolutionary (E) models and the known distance (D), supplemented with multiband photometry. Applying it, we determined the mass of the Cepheid to be 3.41 ± 0.08 M_⊙ and its radius to be 13.65 ± 0.27 R_⊙, and the companion's mass to be 1.89 ± 0.04 M_⊙ and radius to be 12.51 ± 0.62 R_⊙. With the current configuration, the apparent evolutionary age difference of almost 1 Gyr between the components strongly favors the Cepheid merger origin scenario. If so, the actual age of the Cepheid would be 1.09 Gyr, on the edge of Population II stars, indicating that a significant fraction of Cepheids may be much older than typically assumed. We also applied our method to an eclipsing binary Cepheid OGLE-LMC-CEP-1812 with accurately determined physical parameters, obtaining a close agreement, which confirmed our method's reliability.