2025MNRAS.543.2116C

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 J230641.47$+$244055.8. The optical spectrum is dominated by molecular bands from an M-dwarf companion, with additional narrow Balmer and He I lines. The long-term optical light curve folded on its orbital period ($P_\mathrm{orb} = 3.49$ h) shows $\sim 10$ per cent scatter. High-cadence photometry reveals a short-period signal, which we interpret to be the spin period of the WD primary ($P_\mathrm{spin} \simeq 92$ s). The WD spin period is slightly shorter than that of AR Sco ($\rm \sim 117$ 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 $f(M) \simeq 0.2\, {\rm M_\odot }$. The H$\alpha$ 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 $\rm M4.0\pm 0.5$ with $T_\mathrm{\rm eff} \approx 3300$ K. Modelling the stellar contribution constrains the secondary mass ($0.19\, \mathrm{M}_\odot \lesssim M_2\lesssim 0.28\, \mathrm{M}_\odot$), distance ($\simeq 1.2\, {\rm kpc}$), and inclination ($i \simeq 45^\circ-50^\circ$). 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.