2025A&A...702A.263I

Context. RS Oph and T CrB are well-known symbiotic recurrent novae. RS Oph experiences outbursts every 10-20 years, and T CrB roughly every 80 years. Their next outbursts are predicted to take place in several years.
Aims. We monitored spectral variations in these objects to investigate whether there was any prevenient spectroscopic outburst phenomena.
Methods. High- and low-resolution optical spectra were obtained at the Asiago Astrophysical Observatory.
Results. A flare-up of the emission line He II 4686 was observed in April 1994 in a spectrum of RS Oph. This was probably the first sign of the 2006 outburst, because similar but weaker flare-ups followed in October. 1998, August 2001, April 2003, May 2004, and July 2005, namely, with the same interval on a logarithmic scale (one flare-up expected in February 2005 was not observable at our observatory). These flare-ups were possibly related to the instability of the accretion disk. Other signs of impending outbursts are the strengthening of the absorption lines of an A-type star, which occurs about 950 ± 50 days before an outburst; this occurred in 2006, 2021, and probably in 1985. We detected deep absorption lines of A-type star in March 2024 (on JD2460392). We wished to determine whether these absorption lines again strengthened 950 days before the next outburst, the expected date of which is around JD2461342 (October-November 2026). This would mean the interval from the last outburst in 2021 would be only 5 years which would be the shortest in the history of RS Oph. A more precise forecast will be possible if we detect a flare-up of He II 4686 in March 2026 (JD2461116, which would be about 226 days before the expected outburst. The most recent active state of T CrB lasted from December 2014 to around November 2022. The emission line He II 4686 was prominent during the active state, but no flare-up was observed.