2025NatAs...9..672D

Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, with a periodicity of minutes to an hour. These transient radio signals must originate from sources in the Milky Way and from coherent emission processes in astrophysical plasma. They are theorized to be produced in the extreme and highly magnetized environments around white dwarfs or neutron stars. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101 + 5521, whose roughly minute-long pulses arrive with a periodicity of 125.5 min. We find that ILT J1101 + 5521 is an M dwarf-white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101 + 5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101 + 5521 is probably a polar system where magnetic interaction has synchronized the rotational and orbital periods of the white dwarf. Magnetic interaction and plasma exchange between two stars has been theorized to generate sporadic radio emission, making ILT J1101 + 5521 a potential low-mass analogue to such mechanisms.