The re-discovery of the binary star system that created the Nova Scorpii AD 1437 stellar outburst shows that it is now a dwarf nova, suggesting that nova systems spend some time as dwarf novae in between larger outbursts.
‘Cataclysmic variables' are binary star systems in which one star of the pair is a white dwarf, and which often generate bright and energetic stellar outbursts. Classical novae are one type of outburst: when the white dwarf accretes enough matter from its companion, the resulting hydrogen-rich atmospheric envelope can host a runaway thermonuclear reaction that generates a rapid brightening1,2,3,4. Achieving peak luminosities of up to one million times that of the Sun5, all classical novae are recurrent, on timescales of months6 to millennia7. During the century before and after an eruption, the ‘novalike' binary systems that give rise to classical novae exhibit high rates of mass transfer to their white dwarfs8. Another type of outburst is the dwarf nova: these occur in binaries that have stellar masses and periods indistinguishable from those of novalikes9 but much lower mass-transfer rates10, when accretion-disk instabilities11 drop matter onto the white dwarfs. The co-existence at the same orbital period of novalike binaries and dwarf novae—which are identical but for their widely varying accretion rates—has been a longstanding puzzle9. Here we report the recovery of the binary star underlying the classical nova eruption of 11 March AD 1437 (refs 12, 13), and independently confirm its age by proper-motion dating. We show that, almost 500 years after a classical-nova event, the system exhibited dwarf-nova eruptions. The three other oldest recovered classical novae14,15,16 display nova shells, but lack firm post-eruption ages17,18, and are also dwarf novae at present. We conclude that many old novae become dwarf novae for part of the millennia between successive nova eruptions19,20.