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

J191213.72 - 441045.1 is a binary system composed of a white dwarf and an M-dwarf in a 4.03-h 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 various orbital sideband frequencies. Like in the prototype of the class of radio-pulsing white dwarfs, AR Scorpii, the observed pulsed emission seems to be driven by the binary interaction. In this work, we present an analysis of far-ultraviolet spectra obtained with the Cosmic Origins Spectrograph at the Hubble Space Telescope, in which we directly detect the white dwarf in J191213.72 - 441045.1. We find that the white dwarf has a temperature of T_eff = 11485 ± 90 K and mass of 0.59 ± 0.05 M_⊙. We place a tentative upper limit on the magnetic field of ≈50 MG. If the white dwarf is in thermal equilibrium, its physical parameters would imply that crystallization has not started in the core of the white dwarf. Alternatively, the effective temperature could have been affected by compressional heating, indicating a past phase of accretion. The relatively low upper limit to the magnetic field and potential lack of crystallization that could generate a strong field pose challenges to pulsar-like models for the system and give preference to propeller models with a low magnetic field. We also develop a geometric model of the binary interaction which explains many salient features of the system.