Two hot pre-white dwarfs inside the red giant branch planetary nebula Pa 13

Double-core evolution or common-envelope-induced rejuvenation

¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany
² Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁴ Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383, USA
⁵ Instituto de Astrofísica de La Plata, UNLP-CONICET, La Plata, 1900 Buenos Aires, Argentina
⁶ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, D-14476 Potsdam-Golm, Germany
⁷ Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 January 2026
Accepted: 16 March 2026

Abstract

The close binary central stars of planetary nebulae (PNe) offer a unique window for investigating the conditions that immediately follow the ejection of a common envelope (CE). Double-eclipsing and double-lined double systems are particularly valuable as they provide minimally model-dependent constraints on fundamental binary parameters. In this context, we report that the nucleus of Pa 13 (P = 0.3988 d) belongs to this rare class of systems and we present a comprehensive analysis of its double-degenerate binary. We performed a two-component nonlocal thermodynamic equilibrium spectral analysis based on phase-resolved X-shooter spectroscopy, multiband light-curve modeling, spectral energy distribution fitting, and a kinematic analysis. Both stars are found to be hot pre-white dwarfs, with Star 1 being cooler but larger (T_eff = 50.0 kK, R = 0.40 R_⊙) than Star 2 (T_eff = 75.0 kK, R = 0.16 R_⊙). The weakness of the spectral lines of Star 2 made both the atmospheric and radial velocity (RV) analyses challenging, and we uncovered a strong sensitivity of the assumed surface ratio to its derived RV curve. However, the RV curve and Kiel mass of Star 1 (M₁ = 0.41 ± 0.02 M_⊙) could be determined precisely, which allowed for a dynamical mass determination of Star 2 (M₂ = 0.39 ± 0.04 M_⊙). Moreover, we uncovered that Pa 13 exhibits a small but significant orbital eccentricity (e = 0.02 ± 0.01), which makes it only the second post-CE binary central star with a measured eccentricity. Our kinematic analysis shows that Pa 13 belongs to the Galactic halo, implying a system age of ≈11 Gyr. We conclude that Pa 13 provides the strongest evidence so far that PNe can be observed around post-red giant branch stars. Immediately after the CE ejection, Star 1 likely still filled its Roche lobe, which suggests that Pa 13 is a more evolved, detached descendant of over-contact double-degenerate systems such as Hen 2-428. Since the mass ratio of Pa 13 is close to unity, the system may have formed through double-core CE evolution. Alternatively, there must exist an efficient CE-induced rejuvenation mechanism capable of reheating the cool white dwarf in the binary, as already indicated by the Hen 2-428 system.