Orbital Period Changes of Recurrent Nova U Scorpii Demonstrate That <i>M</i> _ejecta = 26 × <i>M</i> _accreted, So U Sco Is Not a Type Ia Supernova Progenitor

Abstract Recurrent nova U Scorpii (U Sco) is one of the prototypes for a Type Ia supernova progenitor. The logic is that the white dwarf is near the Chandrasekhar mass and gas is accumulating onto its surface at a near-maximal accretion rate, so it will soon increase its mass to the supernova trigger. But the white dwarf loses mass every nova eruption, so the issue is balancing the mass ejected ( M ejecta ) against the mass accreted between eruptions ( M accreted ). Measuring M accreted can be done in several ways to usable accuracy. But the old methods for measuring M ejecta (involving the flux in hydrogen emission lines) all have real error bars of 2–3 orders of magnitude. The only solution is to measure the change of the orbital period across the nova eruption (Δ P ). But this solution requires a vast photometric program of eclipse timings stretching decades. For U Sco, a program started in 1989, now reaches its culmination with measures of Δ P for the eruptions of 1999, 2010, 2016, and 2022. This paper reports on 52 new eclipse times (for a total of 218 eclipses from 1945 to 2025), plus a new theory result allowing for the confident calculation of M ejecta from Δ P . The four eruptions ejected a total of (103 ± 14) × 10 −6 M ⊙ , while the white dwarf accreted 4 × 10 −6 M ⊙ over the four previous eruption cycles. With M ejecta = 26 × M accreted , the U Sco white dwarf is losing large masses each eruption cycle, so U Sco can never produce a Type Ia supernova.