Detection of a weak magnetic field in the Balmer emission line white dwarf WDJ1653−1001 Elms, Abbigail K. Bagnulo, Stefano Tremblay, Pier Emmanuel Cunningham, Tim Munday, James Landstreet, John El-Badry, Kareem Caiazzo, Ilaria ; https://orcid.org/0000-0002-4770-5388 Melis, Carl Pinter, Viktoria Weinberger, Alycia ddc:520 The small DAHe and DAe spectral classes comprise isolated, hydrogen-dominated atmosphere white dwarfs that exhibit variable photometric flux and Balmer line emission. These mysterious systems offer unique insight into the complex interplay between magnetic fields, stellar rotation and atmospheric activity in single white dwarfs. DAHe stars have detectable magnetic fields through Zeeman-split spectral lines, whereas DAe stars lack such splitting. We report the first discovery and characterization of magnetism in the DAe white dwarf WD J165335.21−100116.33 with new time-resolved spectropolarimetry from FORS2. We detect a weak but variable longitudinal magnetic field with values Bz > −9.2 ± 2.4 kG and Bz < −2.2 ± 1.0 kG. Independent ZTF and ATLAS photometry reveal a consistent period of P = 80.3070 ± 0.0007 h. Time-resolved optical spectroscopy obtained with six ground-based instruments demonstrates strong modulation in the strength of the Hα and Hβ Balmer line emission with P = 80.2922 ± 0.0108 h. The photometric flux and Balmer emission strength vary in antiphase, with the strongest magnetic detections coinciding with phases of low photometric flux and strong line emission. These characteristicssupport the theory that a magnetically active, temperature-inverted spot/region is producing an optically thin chromospheric emission region. Comparison with other DAe and DAHe white dwarfsreveals all systems have a strikingly similar antiphase phenomenology, reinforcing the theory that they are subject to a unified physical mechanism. With the detection of a weak magnetic field, we reclassify WD J165335.21−100116.33 as a low-field DAHe white dwarf. Oxford University Press 2026 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/21745 https://research-explorer.ista.ac.at/download/21745/21794 Elms AK, Bagnulo S, Tremblay PE, et al. Detection of a weak magnetic field in the Balmer emission line white dwarf WDJ1653−1001. <i>Monthly Notices of the Royal Astronomical Society</i>. 2026;548(1). doi:<a href="https://doi.org/10.1093/mnras/stag505">10.1093/mnras/stag505</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1093/mnras/stag505 info:eu-repo/semantics/altIdentifier/issn/0035-8711 info:eu-repo/semantics/altIdentifier/e-issn/1365-2966 info:eu-repo/semantics/altIdentifier/arxiv/2603.12048 https://creativecommons.org/licenses/by/4.0/ info:eu-repo/semantics/openAccess