A sibling of AR Scorpii: SDSS J230641.47+244055.8 and the observational blueprint of white dwarf pulsars

Radio pulsating white dwarf (WD) systems, known as WD pulsars, are binary systems where the rapidly spinning WD interacts with a low-mass companion, producing pulsed non-thermal emission observed across the entire electromagnetic spectrum. Only two such systems are known: AR Sco and eRASSU J191213.9-441044. Here, we present the discovery of a third WD pulsar, SDSS J230641.47244055.8. The optical spectrum is dominated by molecular bands from an M-dwarf companion, with additional narrow Balmer and He i lines. The long-term optical light curve folded on its orbital period (h) shows per cent scatter. High-cadence photometry reveals a short-period signal, which we interpret to be the spin period of the WD primary (s). The WD spin period is slightly shorter than that of AR Sco (s), the WD pulsar prototype. Time-resolved spectroscopy reveals emission from the irradiated companion and Na i absorption tracing its centre of mass, yielding a binary mass function of. The H emission includes a low-amplitude broad component, resembling the energetic flashes seen in AR Sco. Using spectral templates, we classify the companion to be most likely a with K. Modelling the stellar contribution constrains the secondary mass (), distance (), and inclination (). We discuss the proposed evolutionary scenarios and summarize the observational properties of all three known WD pulsars, establishing a benchmark for identifying and classifying future members of this class.