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Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce x-ray emission, but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray emission (0.1 to 10 billion electron volts) from the recently detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary and that particles can be accelerated effectively to produce π° decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.

We present the results of spectroscopic observations of nine symbiotic star candidates in the 360–735 nm wavelength range carried out with the TDS spectrograph of the 2.5-m SAI MSU CMO telescope. Eight selected sample stars were classified as red giants of the M3–M6 spectral types. Light curve analysis revealed that all nine stars can be attributed to SRA- and SRB-type variables (based on the classification of the General Catalog of Variable Stars–GCVS). It is shown that one of the stars, V520 And, can be classified as a symbiotic star. Its spectrum reveals H I, He I, Fe II emission lines, weak lines of [O III] 5007, [Ne III] 3869, and He II 4686, and a Balmer jump in emission. The emission spectrum is found to be variable on both the long and very short timescales. A high-resolution spectrum ( ) obtained with the new CMO spectrograph showed the double-peaked shape of the H line and the presence of an absorption component in it. The October 14, 2023 photometric monitoring revealed a flickering in the band with an amplitude of on a timescale of 10–15 minutes. The luminosity, effective temperature, and radius of the V520 And cool component were determined: , K, and , respectively. Evidence for the presence in the system of an accretion disk with the luminosity was also found.

Symbiotic stars are interacting binary systems composed of an evolved star (generally a late-type red giant) and a degenerate or dwarf companion in orbit close enough for mass transfer to occur. Understanding the status of the late-type star is important for developing binary models for the symbiotic systems as it affects the transfer of matter needed to activate the hot component. Infrared observations have been very useful in probing the nature of late-type stars in symbiotic systems. This work presents a set of symbiotic stars observed with SOAR/OSIRIS (R∼3000) in the H-band. We aimed to search for possible molecular circumstellar emission, to characterize the cool companion in these systems, and to confront the new findings with those obtained from the previous K-band classifications. We detected molecular emission from just one object, BI Cru, which displays the second-overtone CO-bands. To fit the observed photospheric CO absorption bands, we used the MARCS atmosphere models. We present our results as a mini atlas of symbiotic stars in the near-infrared region to facilitate the comparison among different observed symbiotic systems.

The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, x-ray binaries, and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. More than 70% of all massive stars will exchange mass with a companion, leading to a binary merger in one-third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These datasets have transformed the search for new symbiotic candidates, providing extensive time-domain information crucial for their classification and analysis. This review highlights recent observational results that have expanded the known population of symbiotic stars, refined classification criteria, and enhanced our understanding of their variability. Despite these advances, fundamental questions remain regarding their long-term evolution, mass transfer and accretion processes, or their potential role as progenitors of Type Ia supernovae. With ongoing and upcoming surveys, the coming years promise new discoveries and a more comprehensive picture of these intriguing interacting systems.

This study is based on photometric observations of ZZ CMi during the last observational season (2024/2025). Other than looking for flickering we were interested in following the variability of brightness in the same period. In the presented multicolor photometry, we find variability throughout each night — pronounced flickering in the U band. The amplitude of the flickering is about 0.22−0.48 mag in the U band. In the B band, the variability is lower, with an amplitude ≤0.05 mag. The work shows multiple convenient standard stars around the symbiotic star ZZ CMi.

We report on high-resolution spectra of the two recurring novae, T CrB and RS Oph, obtained in 2004 when no outbursts were in progress. Selected regions of the spectra between 6500 and 8800 Å were measured for equivalent widths and analyzed for metallicity. Lines of Fe I, Ni I, Si I, and Ti Iwere used to establish the effective temperature. The metallicity as derived using models is near solar with an uncertainty estimated to be near a factor of 2 for both stars. Both stars show a strong lithium line at 6707.8 Å. Approximate Li abundances were derived using model atmospheres for a direct comparison with the nearby Fe Iline at 6710.31 Å and the Ca Iground level line at 6572.78 Å. The Li abundances are nearlog N(Li) = 1.2 log N ( Li ) = 1.2 for RS Oph and 0.8 for T CrB on the scale oflog N(H) = 12.0 log N ( H ) = 12.0 . Such Li abundances are high for single K and M giants. A survey of symbiotic stars with cool components of types K and M showed no recognizable Li line in 28 stars with high quality spectra. This makes the two repeating novae different from the other symbiotics that consist of a red giant and a white dwarf.

We report the mathematical representation of the so called eccentric eclipse model, whose numerical solutions can be used to obtain the physical parameters of a quiescent eclipsing symbiotic system. Indeed the nebular region produced by the collision of the stellar winds should be shifted to the orbital axis because of the orbital motion of the system. This mechanism is not negligible, and it led us to modify the classical concept of an eclipse. The orbital elements obtained from spectroscopy and photometry of the symbiotic EG Andromedae were used to test the eccentric eclipse model. Consistent values for the unknown orbital elements of this symbiotic were obtained. The physical parameters are in agreement with those obtained by means of other simulations for this system.

Symbiotic stars are interacting binaries in which the first-formed white dwarf accretes and burns material from a red giant companion. This paper aims at presenting physical characteristics of these objects and discussing their possible link with progenitors of Type Ia supernovae.