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J191213.72-441045.1 is a binary system composed of a white dwarf and anM-dwarf in a 4.03-hour orbit. It shows emission in radio, optical, and X-ray,all modulated at the white dwarf spin period of 5.3 min, as well as variousorbital sideband frequencies. Like in the prototype of the class ofradio-pulsing white dwarfs, AR Scorpii, the observed pulsed emission seems tobe driven by the binary interaction. In this work, we present an analysis offar-ultraviolet spectra obtained with the Cosmic Origins Spectrograph at theHubble Space Telescope, in which we directly detect the white dwarf inJ191213.72-441045.1. We find that the white dwarf has an effective temperatureof 11485+/-90 K and mass of 0.59+/-0.05 solar masses. We place a tentativeupper limit on the magnetic field of ~50 MG. If the white dwarf is in thermalequilibrium, its physical parameters would imply that crystallisation has notstarted in the core of the white dwarf. Alternatively, the effectivetemperature could have been affected by compressional heating, indicating apast phase of accretion. The relatively low upper limit to the magnetic fieldand potential lack of crystallisation that could generate a strong field posechallenges to pulsar-like models for the system and give preference topropeller models with a low magnetic field. We also develop a geometric modelof the binary interaction which explains many salient features of the system.

Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations