A Second Candidate Magnetic Helium Core White Dwarf and 3 Other Variable White Dwarfs in the Globular Cluster NGC 6397

Using archival Hubble Space Telescope observations, we report the discovery of four variable low-mass white dwarfs (WDs; 0.18 M ⊙ ≤ M ≤ 0.5 M ⊙ ) in the globular cluster NGC 6397. One source exhibits a periodic optical modulation of 5.21 ± 0.02 hr, which we interpret as potentially due to the rotation of a magnetic helium core WD (He WD). This makes this candidate the second magnetic He WD in NGC 6397, and one of the few He WDs with a measured rotation period. The other three candidates show aperiodic variability, with a change in magnitude ranging from ∼0.11 to 0.6. These discoveries highlight the importance of high-cadence photometric surveys in dense stellar environments. Follow-up spectroscopic observations are needed to confirm the nature of the variability of these systems.