  Type Ia supernovae (SNe Ia) are thermonuclear explosions of degenerate white
dwarf (WD) stars destabilized by mass accretion from a companion star, but the
nature of their progenitors remains poorly understood. A way to discriminate
between progenitor systems is through radio observations; a non-degenerate
companion star is expected to lose material through winds or binary interaction
prior to explosion, and the SN ejecta crashing into this nearby circumstellar
material (CSM) should result in radio synchrotron emission. However, despite
extensive efforts, no SN Ia has ever been detected at radio wavelengths, which
suggests a clean environment and a companion star that is itself a degenerate
WD star. Here we report on the study of SN 2020eyj, a SN Ia showing helium-rich
CSM, as revealed by its spectral features, infrared emission and, for the first
time in a SN Ia, a radio counterpart. Based on our modeling, we conclude the
CSM likely originates from a single-degenerate (SD) binary system where a WD
accretes material from a helium donor star, an often hypothesized formation
channel for SNe Ia. We describe how comprehensive radio follow-up of SN
2020eyj-like SNe Ia can improve the constraints on their progenitor systems.
