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AR Scorpii (AR Sco) is the only radio-pulsing white dwarf known to date. Itshows a broad-band spectrum extending from radio to X-rays whose luminositycannot be explained by thermal emission from the system components alone, andis instead explained through synchrotron emission powered by the spin-down ofthe white dwarf. We analysed NTT/ULTRACAM, TNT/ULTRASPEC, and GTC/HiPERCAMhigh-speed photometric data for AR Sco spanning almost seven years and obtaineda precise estimate of the spin frequency derivative, now confirmed with50-sigma significance. Using archival photometry, we show that the spin downrate of P/Pdot = 5.6e6 years has remained constant since 2005. As well asemploying the method of pulse-arrival time fitting used for previous estimates,we also found a consistent value via traditional Fourier analysis for the firsttime. In addition, we obtained optical time-resolved spectra with WHT/ISIS andVLT/X-shooter. We performed modulated Doppler tomography for the first time forthe system, finding evidence of emission modulated on the orbital period. Wehave also estimated the projected rotational velocity of the M-dwarf as afunction of orbital period and found that it must be close to Roche lobefilling. Our findings provide further constraints for modelling this uniquesystem.

Long-term photometric monitoring and spectroscopy of the white dwarf pulsar AR Scorpii