HIP 15429: A newborn Be star on an eccentric binary orbit

Interaction in close binary systems is common in massive stars. Typically, the mass donor is stripped of its hydrogen envelope and evolves to become a hot helium star, while the accretor gains mass and angular momentum, spinning up in the process. However, the small number of well-constrained post-interaction binary systems currently limits detailed comparisons with binary evolution models. We have identified a new post-interaction binary, HIP 15429, consisting of a stripped star and a recently formed rapidly rotating Be star companion (v_rotsini ≈ 270 km/s) that shares many similarities with recently identified bloated stripped stars. Based on the orbital fitting of multi-epoch radial velocities, we find a 221-day binary period. We also find an eccentricity of e = 0.52, which is unexpectedly high, as tides are expected to have circularised the orbit efficiently during the presumed recent mass transfer. The formation of a circumbinary disc during the mass-transfer phase or the presence of an unseen tertiary companion might explain the orbit’s high eccentricity. We determined the physical parameters for both stars in HIP 15429 by fitting the spectra of the disentangled binary components and multi-band photometry. The stripped nature of the donor star is affirmed by its high luminosity at a low inferred mass (≲1 M_⊙) and the imprints of CNO-processed material on the surface abundances. The donor’s large radius and cool temperature (T_eff = 13.5 ± 0.5 kK) suggest that it has only recently ceased mass transfer. Evolutionary models assuming a 5–6 M_⊙ progenitor can reproduce these parameters, and they imply that the binary is currently evolving towards a stage where the donor becomes a subdwarf orbiting a Be star. The remarkably high eccentricity of HIP 15429 challenges standard tidal evolution models, suggesting either inefficient tidal dissipation or external influences, such as a tertiary companion or circumbinary disc. This underscores the need to identify and characterise more post-mass transfer binaries to benchmark and refine theoretical models of binary evolution.