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FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediatepolar) went into a low-state in 2016, from which it slowly and steadilyrecovered without showing dwarf nova outbursts. This requires explanation sincein a low-state, the mass-transfer rate is in principle too low for the disc tobe fully ionized and the disc should be subject to the standard thermal andviscous instability observed in dwarf novae. We investigate the conditionsunder which an accretion disc in an intermediate polar could exhibit aluminosity drop of 2 magnitudes in the optical band without showing outbursts.We use our numerical code for the time evolution of accretion discs, includingother light sources from the system (primary, secondary, hot spot). We showthat although it is marginally possible for the accretion disc in the low-stateto stay on the hot stable branch, the required mass-transfer rate in the normalstate would then have to be extremely high, of the order of 10$^{19}$ gs$^{-1}$or even larger. This would make the system so intrinsically bright that itsdistance should be much larger than allowed by all estimates. We show thatobservations of FO Aqr are well accounted for by the same mechanism that wehave suggested as explaining the absence of outbursts during low states of VYScl stars: during the decay, the magnetospheric radius exceeds thecircularization radius, so that the disc disappears before it enters theinstability strip for dwarf nova outbursts. Our results are unaffected, andeven reinforced, if accretion proceeds both via the accretion disc and directlyvia the stream during some intermediate stages; the detailed process throughwhich the disc disappears still needs investigations.

The disappearance and reformation of the accretion disc during a low state of FO Aquarii