Aims. We determine the temporal evolution of the luminosity (L WD ), radius (R WD ) and effective temperature (T eff ) of the white dwarf (WD) pseudophotosphere of V339 Del from its discovery to around day 40. Another main objective was studying the ionization structure of the ejecta. Methods. These aims were achieved by modelling the optical/near-IR spectral energy distribution (SED) using low-resolution spectroscopy (3500-9200 Å), UBVR C I C and JHKLM photometry. Important insights in the physical conditions of the ejecta were gained from an analysis of the evolution of the Hα and Raman-scattered 6825 Å O vi line using medium-resolution spectroscopy (R ∼ 10 000). Results. During the fireball stage (Aug. 14. 8-19.9, 2013), T eff was in the range of 6000-12 000 K, R WD was expanding non-uniformly in time from ∼66 to ∼300 (d/3 kpc) R , and L WD was super-Eddington, but not constant. Its maximum of ∼9 × 10 38 (d/3 kpc) 2 erg s −1 occurred around Aug. 16.0, at the maximum of T eff , half a day before the visual maximum. After the fireball stage, a large emission measure of 1.0−2.0×10 62 (d/3 kpc) 2 cm −3 constrained the lower limit of L WD to be well above the super-Eddington value. The mass of the ionized region was a few ×10 −4 M , and the mass-loss rate was decreasing from ∼5.7 (Aug. 22) to ∼0.71× 10 −4 M yr −1 (Sept. 20).The evolution of the Hα line and mainly the transient emergence of the Raman-scattered O vi 1032 Å line suggested a biconical ionization structure of the ejecta with a disk-like H i region persisting around the WD until its total ionization, around day 40. On Sept. 20 (day 35), the model SED indicated a dust emission component in the spectrum. The dust was located beyond the H i zone, where it was shielded from the hard, > ∼ 10 5 K, radiation of the burning WD at that time. Conclusions. Our extensive spectroscopic observations of the classical nova V339 Del allowed us to map its evolution from the very early phase after its explosion. It is evident that the nova was not evolving according to the current theoretical prediction. The unusual non-spherically symmetric ejecta of nova V339 Del and its extreme physical conditions and evolution during and after the fireball stage represent interesting new challenges for the theoretical modelling of the nova phenomenon.

Context. AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 years, during June of 2015, it erupted again showing characteristics of the Z And-type outburst. Aims. The primary objective is to determine basic characteristics, the nature and type of the 2015 outburst of AG Peg. Methods. We achieved this aim by modelling the spectral energy distribution using low-resolution spectroscopy (330-750 nm; R = 500-1000), medium-resolution spectroscopy (420-720 nm; R ∼ 11000), and U BVR C I C photometry covering the 2015 outburst with a high cadence. Optical observations were complemented with the archival HST and FUSE spectra from the preceding quiescence. Results. During the outburst, the luminosity of the hot component was in the range of 2-11×10 37 (d/0.8 kpc) 2 erg s −1 , being in correlation with the light curve (LC) profile. To generate the maximum luminosity by the hydrogen burning, the white dwarf (WD) had to accrete at ∼ 3 × 10 −7 M ⊙ yr −1 , which exceeds the stable-burning limit and thus led to blowing optically thick wind from the WD. We determined its mass-loss rate to a few ×10 −6 M ⊙ yr −1 . At the high temperature of the ionising source, 1.5 − 2.3 × 10 5 K, the wind converted a fraction of the WD's photospheric radiation into the nebular emission that dominated the optical. A one order of magnitude increase of the emission measure, from a few ×10 59 (d/0.8 kpc) 2 cm −3 during quiescence, to a few ×10 60 (d/0.8 kpc) 2 cm −3 during the outburst, caused a 2 mag brightening in the LC, which is classified as the Z And-type of the outburst. Conclusions. The very high nebular emission and the presence of a disk-like H i region encompassing the WD, as indicated by a significant broadening and high flux of the Raman-scattered O vi 6825 Å line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.

The disk surrounding the primary component of π Aqr nearly disappeared in early 2014.• The disk has slowly recovered, now reaching strengths not seen in three decades.• This evolution in line strength is accompanied by changes in disk structure.Evolution of the disk of π Aqr: from near-disappearance to a strong maximum Abstract Some Be stars display important variability of the strength of the emission lines formed in their disk. This is notably the case of π Aqr. We present here the recent evolution of the Be disk in this system thanks to spectra collected by amateur spectroscopists since the end of 2013. A large transition occurred: the emission linked to the Be disk nearly disappeared in January 2014, but the disk has recovered, with a line strength now reaching levels only seen during the active phase of . In parallel to this change in strength occurs a change of disk structure, notably involving the disappearance of the strong asymmetry responsible for the V/R modulation.

Context. RRab stars are large amplitude pulsating stars in which the pulsation wave is a progressive wave. Consequently, strong shocks, stratification effects, and phase lag may exist between the variations associated with line profiles formed in different parts of the atmosphere, including the shock wake. The pulsation is associated with a large extension of the expanding atmosphere, and strong infalling motions are expected. Aims. The objective of this study is to provide a general overview of the dynamical structure of the atmosphere occurring over a typical pulsation cycle. Methods. We report new high-resolution observations with suitable time resolution of Hα and sodium lines in the brightest RR Lyrae star of the sky: RR Lyr (HD 182989). A detailed analysis of line profile variations over the whole pulsation cycle is performed to understand the dynamical structure of the atmosphere. Results. The main shock wave appears when it exits from the photosphere at ϕ 0.89, i.e., when the main Hα emission is observed. Whereas the acceleration phase of the shock is not observed, a significant deceleration of the shock front velocity is clearly present. The radiative stage of the shock wave is short: 4% of the pulsation period (0.892 < ϕ < 0.929). A Mach number M > 10 is required to get such a radiative shock. The sodium layer reaches its maximum expansion well before that of Hα (∆ϕ = 0.135). Thus, a rarefaction wave is induced between the Hα and sodium layers. A strong atmospheric compression occurring around ϕ = 0.36, which produces the third Hα emission, takes place in the highest part of the atmosphere. The region located lower in the atmosphere where the sodium line is formed is not involved. The amplification of gas turbulence seems mainly due to strong shock waves propagating in the atmosphere rather than to the global compression of the atmosphere caused by the pulsation. It has not yet been clearly established whether the microturbulence velocity increases or decreases with height in the atmosphere. Furthermore, it seems very probable that an interstellar component is visible within the sodium profile.

Context. The so-called Hα third emission occurs around pulsation phase ϕ = 0.30. It has been observed for the first time in 2011 in some RR Lyrae stars. The emission intensity is very weak, and its profile is a tiny persistent hump in the red side-line profile. Aims. We report the first observation of the Hα third emission in RR Lyr itself (HD 182989), the brightest RR Lyrae star in the sky. Methods. New spectra were collected in 2013−2014 with the AURELIE spectrograph (resolving power R = 22 700, T152, Observatoire de Haute-Provence, France) and in 2016−2017 with the eShel spectrograph (R = 11 000, T035, Observatoire de Chelles, France). In addition, observations obtained in 1997 with the ELODIE spectrograph (R = 42 000, T193, Observatoire de Haute-Provence, France) were reanalyzed. Results. The Hα third emission is clearly detected in the pulsation phase interval ϕ = 0.188−0.407, that is, during about 20% of the period. Its maximum flux with respect to the continuum is about 13%. The presence of this third emission and its strength both seem to depend only marginally on the Blazhko phase. The physical origin of the emission is probably due to the infalling motion of the highest atmospheric layers, which compresses and heats the gas that is located immediately above the rising shock wave. The infalling velocity of the hot compressed region is supersonic, almost 50 km s −1 , while the shock velocity may be much lower in these pulsation phases. Conclusions. When the Hα third emission appears, the shock is certainly no longer radiative because its intensity is not sufficient to produce a blueshifted emission component within the Hα profile. At phase ϕ = 0.40, the shock wave is certainly close to its complete dissipation in the atmosphere.