# SALT HRS discovery of a long period double-degenerate binary in the planetary nebula NGC 1360

## TL;DR

This paper reports the discovery of a long-period (142 days) binary central star in the planetary nebula NGC 1360, challenging existing models that predict shorter periods for post-common-envelope systems.

## Contribution

First systematic SALT HRS survey detection of a long-period binary in a planetary nebula, providing new insights into binary evolution and nebula features.

## Key findings

- Discovered a 142-day orbital period binary in NGC 1360.
- NGC 1360 shows features consistent with post-CE evolution.
- Long-period binaries may be more common than models suggest.

## Abstract

Whether planetary nebulae (PNe) are predominantly the product of binary stellar evolution as some population synthesis models (PSM) suggest remains an open question. Around 50 short period binary central stars ($P\sim1$ d) are known, but with only four with measured orbital periods over 10 d, our knowledge is severely incomplete. Here we report on the first discovery from a systematic SALT HRS survey for long period binary central stars. We find a 142 d orbital period from radial velocities of the central star of NGC~1360, HIP~16566. NGC~1360 appears to be the product of common-envelope (CE) evolution, with nebula features similar to post-CE PNe, albeit with an orbital period considerably longer than expected to be typical of post-CE PSM. The most striking feature is a newly-identified ring of candidate low-ionisation structures (LIS). Previous spatio-kinematic modelling of the nebula gives a nebula inclination of $30\pm10$ deg, and assuming the binary nucleus is coplanar with the nebula, multi-wavelength observations best fit a more massive, evolved WD companion. A WD companion in a 142 d orbit is not the focus of many PSM, making NGC~1360 a valuable system with which to improve future PSM work. HIP~16566 is amongst many central stars in which large radial velocity variability was found by low-resolution surveys. The discovery of its binary nature may indicate long period binaries may be more common than PSM models predict.