Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

¹ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
² Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁴ Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, CA 92093, USA

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

Received: 16 December 2025
Accepted: 5 February 2026

Abstract

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a high number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16 000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with the characteristics of an extremely low-mass He-core WD. This is further supported by the mass of 0.23 ± 0.03 M_⊙ obtained from fitting the star’s spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 M_⊙ for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50°. Some significant residuals in radial velocity remain with our best orbital solution, and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.