2023A&A...680L..18Z

Context. T CrB (NOVA CrB 1946) is a famous recurrent nova with a recurrence timescale of 80 years.
Aims. We aim to estimate the colours, luminosity, and mass-accretion rate for T CrB (NOVA CrB 1946) during and after the superactive state.
Methods. We performed and analysed UBV photometry of the recurrent nova T CrB.
Results. For the hot component of T CrB, we find average dereddened colours of (U - B)₀ = -0.70 ± 0.08 and (B - V)₀ = 0.23 ± 0.06, which correspond to an effective temperature of 9400 ± 500 K and an optical luminosity of 40 - 110 L_⊙ during the superactive state (2016-2022). After the end of the superactive state, the hot component became significantly redder, (U - B)₀ ≈ -0.3 and (B - V)₀ ≈ 0.6 in August 2023, and its luminosity decreased markedly to 20 - 25 L_⊙ in April-May 2023, and to 8 - 9 L_⊙ in August 2023. The total mass accreted during the superactive state from 2014 to 2023 is ∼2 × 10^–7 M_⊙.
Conclusions. This is a significant fraction of the mass required to cause a thermonuclear runaway (TNR). Overall our results support a model in which a large accretion disc acts as a reservoir with increased accretion rate onto the central white dwarf during disc high states, ultimately leading to a TNR explosion, which now seems to be imminent.