Circumbinary disc interactions and stochastic dust obscuration in the post-asymptotic-giant-branch binary HD 213985

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
² Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
³ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Brussels, Belgium
⁴ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Rabiańska 8, 87-100 Toruń, Poland
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^⋆ Corresponding author: ilkiewicz@camk.edu.pl

Received: 22 December 2024
Accepted: 17 April 2025

Abstract

HD 213985 is an eccentric binary system with a post-AGB primary and a jet-launching secondary star. We confirm that the system photometric variability is likely due to obscuration by the inner edge of a circumbinary disc, similar to RVb-type RV Tau stars. The system has shown an increase in the orbital variability amplitude in optical photometric bands, along with irregular changes in its shape that often started to appear skewed. Variability in the Na D lines suggests that this behaviour may be driven by interactions between the circumbinary disc and outflows through the L2 Lagrange point. Moreover, HD 213985 has exhibited episodes of short-term fluctuations whose appearance is not strictly related to the orbital phase. This variability is consistent with obscuration by transient dust structure leading to weather-like variability patterns.