A TESS view of post-eruption variability in novae V1405 Cas, V1716 Sco, and V1674 Her

We analyzed TESS archival data of three novae after recent outbursts, searching for the orbital and white dwarf (WD) rotation period and possible variations in these periods. In V1405 Cas, we detected a period of ∼116.88 seconds, which we identified as due to the WD spin, and measured a rate of increase of 0.00165 ± 0.000006 s d −1 , which is one of the fastest spin-down rates ever recorded. The rapid spin-down coupled with an X-ray luminosity several orders of magnitude lower than the available spin-down power, strongly indicates that the system is in a magnetic “propeller” state, which means the rotational energy powers the system’s X-ray luminosity. We measured a previously unknown orbital period of 1.357 ± 0.005 days for V1716 Sco. If the X-ray flux modulation with a period of 77.9 s detected in outburst for this nova is due to the rotation of a strongly magnetized white dwarf, which is the case in other novae with similar modulations of the supersoft X-ray source in outburst, the system is in a parameter space that challenges standard models of cataclysmic variable evolution. For V1674 Her, which has already been classified as an intermediate polar (IP), we confirm the known spin period of 501.328 ± 0.024 s and the orbital period of 0.15293 ± 0.00004 days. These suggest that the spin modulation was also the root cause of the periodicity in X-rays in outburst, and that the WD atmosphere in the supersoft X-ray phase was not thermally homogeneous. Our results highlight the power of high-cadence, continuous observations in revealing extreme and unexpected characteristics of accreting white dwarfs.