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The intermediate polar FO Aquarii (FO Aqr) experienced its first reported low-accretion states in 2016, 2017, and 2018, and, using newly available photographic plates, we identify prediscovery low states in 1965, 1966, and 1974. The primary focus of our analysis, however, is an extensive set of time-series photometry obtained between 2002 and 2018, with particularly intensive coverage of the 2016–2018 low states. After computing an updated spin ephemeris for the white dwarf (WD), we show that its spin period began to increase in 2014 after having spent 27 yr decreasing; no other intermediate polar has experienced a sign change of its period derivative, but FO Aqr has now done so twice. Our central finding is that the recent low states all occurred shortly after the WD began to spin down, even though no low states were reported in the preceding quarter-century, when it was spinning up. Additionally, the system's mode of accretion is extremely sensitive to the mass-transfer rate, with accretion being almost exclusively disk-fed when FO Aqr is brighter than V ∼ 14 and substantially stream-fed when it is not. Even in the low states, a grazing eclipse remains detectable, confirming the presence of a disklike structure (but not necessarily a Keplerian accretion disk). We relate these various observations to theoretical predictions that during the low state, the system's accretion disk dissipates into a non-Keplerian ring of diamagnetic blobs. Finally, a new XMM-Newton observation from a high state in 2017 reveals an anomalously soft X-ray spectrum and diminished X-ray luminosity compared to pre-2016 observations. [ABSTRACT FROM AUTHOR]

We present a time-series analysis of the asynchronous polar CD Ind, using fast cadence TESS photometry. A similar analysis is performed using ground based photometry of BY Cam. These asynchronous polars show remarkably similar light curves and enigmatic timing characteristics. A pair of competing models for the assignment of the white dwarf spin period have been presented for both binaries. TESS allows for the breaking of this degeneracy by providing continuous coverage over several beat-cycles. The CD Ind light curve displays a super-orbital period of 7.2 days due to beat-phased modulation of the accretion rate onto a permanently visible spot. This (main) accretion region is active for 3/4 of the beat-cycle. The dominant photometric signal is at 109.654(4) min and is identified as a side-band related to the white dwarf spin and the binary orbit, analogous to that found in BY Cam. We obtain the white dwarf spin period of CD Ind as 110.820(5) min and an orbital period of 111.952(8) min. The TESS light curve supports a pole-switching scenario for CD Ind, with a total of 4 alternating, and oppositely positioned accretion regions. This inclination of CD Ind is estimated as i = 65 ° ± 10 °. One accretion region remains in view of the observer at all times. In addition, two pulsed accretion spots each accrete for just less than 1/2 of the beat-cycle. These alternating spots are orthogonal to the main accretion region (which is always in view), and are found to be roughly 180 ° apart in longitude. Finally, since the mean flux drops by a factor of 2 for about 1/4 of the beat-cycle and the dominant accretion region turns off during this time, we hypothesize the existence of a fourth (permanently hidden) accretion region opposite to the one permanently in view. Photometric models for the 2- and 4-pole accretion configurations are developed and only the 4-pole model is consistent with the observed periodogram for CD Ind. We conclude that complex magnetic fields affect accretion flow onto these white dwarf stars. In particular, the magnetic field of the white dwarf in both of these binaries is inconsistent with either a centered or an offset dipole configuration. [ABSTRACT FROM AUTHOR]