The reduction of all the available photometry for the Cepheid CC Lyr has allowed its $$O-C$$ diagram spanning a time interval of 124 years to be constructed. The data obtained provide evidence for the existence of large random fluctuations in the period ( $$\varepsilon/P\approx 0.046$$ ). A formal fitting of the $$O-C$$ residuals has made it possible to estimate the rate of secular increase in the period, $$dP/dt=229.3({\pm}{14.8})$$ s yr $${}^{-1}$$ . The pulsation stability test proposed by Lombard and Koen (1993) has confirmed that the increase in the period is real.

The formation of the rubidium resonance lines is considered by taking into account the effects of departures from local thermodynamic equilibrium (LTE). A rubidium model atom has been constructed using 29 Rb I levels and the Rb II ground level. Non-LTE calculations have been performed for a grid of model atmospheres with $$T_{\textrm{eff}}$$ from 3500 to 6500 K, $$\log g$$ from 1.0 to 5.0, [Fe/H] from $${-}1.0$$ to $${+}0.5$$ , $$V_{t}=1.0$$ km s $${}^{-1}$$ , and a relative rubidium abundance $$\textrm{[Rb/Fe]}=0.0$$ . It is shown that disregarding the non-LTE effects can lead to significant errors in the abundance of this element. The non-LTE corrections for dwarf stars with effective temperatures below 4000 K depend critically on the inclusion of collisional interactions with hydrogen atoms. The differences in rubidium abundance when using quantum-mechanical calculations and Drawin’s theoretical approximation to take into account the collision rates of atoms with hydrogen atoms can reach 0.17 dex. The rubidium abundance has been determined from its lines in the solar spectrum, $$\textrm{(Rb/H)}=2.35\pm 0.05$$ , which virtually coincides with the rubidium abundance deduced from the analysis of meteorites, $$\textrm{(Rb/H)}=2.36\pm 0.03$$ .

Evidence of wind variability and velocity stratification in the extended atmosphere has been found in the spectra of the supergiant V340 Ser ( $${=}$$ IRAS 17279 $$-$$ 1119) taken at the 6-m BTA telescope with a resolution $$R\geq 60\,000$$ . The H $$\alpha$$ line has a P Cyg profile whose absorption component ( $$V_{\odot}={+}34$$ km s $${}^{-1}$$ ) is formed in the upper layers of the expanding atmosphere close the circumstellar medium. For four dates the mean velocity has been derived from the positions of 300–550 symmetric metal absorptions with an accuracy better than $${\pm}0.1$$ km s $${}^{-1}$$ : $$V_{\odot}=59.30$$ , $$60.09$$ , $$58.46$$ , and $$55.78$$ km s $${}^{-1}$$ . A lot of low-excitation metal lines have an inverse P Cyg profile. The mean positions of their emission components, $$V_{\odot}=46.3\pm 0.4$$ km s $${}^{-1}$$ , differ systematically from the velocity inferred from symmetric absorptions, suggesting the presence of a velocity gradient in the supergiant’s extended atmosphere. The multicomponent profile of the Na I D lines contains the interstellar, $$V_{\odot}={-}11.2$$ km s $${}^{-1}$$ , and circumstellar, $$V_{\odot}={+}10$$ km s $${}^{-1}$$ , components and the component forming in the upper atmospheric layers, $$V_{\odot}={+}34.0$$ km s $${}^{-1}$$ . The mean velocity from 20–30 diffuse interstellar bands (DIBs) identified in the spectra, $$V_{\odot}\textrm{(DIBs)}={-}11.6\pm 0.2$$ km s $${}^{-1}$$ , agrees with the velocity from interstellar Na I and K I components. The equivalent width of the oxygen triplet $$\textrm{W(O\,I 7774)}=1.25$$ Å corresponds to an absolute magnitude of the star $$Mv\approx{-}4.6^{m}$$ , which, given the total (interstellar $$+$$ circumstellar) extinction, leads to a distance to the star $$d\approx 2.3$$ kpc.

Based on observations with the new transient double-beam spectrograph (TDS) at the 2.5-m telescope of the Caucasus Mountain Observatory of SAI MSU, we have determined the types and found the redshifts for six new X-ray sources (four quasars and two galaxy clusters) detected by the SRG space observatory during the Lockman Hole observations at the eROSITA performance verification phase. We show that the TDS allows the spectra of objects $${\sim}20^{m}$$ to be taken in 2 h of observations with a signal-to-noise ratio higher than 5 and a resolution $$R\sim 1500$$ . The types and redshifts of the objects determined from our spectroscopic observations agree well with the predictions based on photometric data using the SRGz automatic classification system.

We have solved the problem of the acceleration of ions (protons) incident on the front of a nonlinear whistler for which the structure of its electromagnetic fields is assumed to be known and defined by the relations derived by us previously. We have established the dependence of the energy to which the protons are accelerated on the angle between the whistler propagation direction and the direction of the external magnetic field and on the whistler speed. The results obtained are applied to the Earth’s bow shock by assuming that the bow shock front has the structure of a nonlinear whistler. We show that the protons are accelerated at the Earth’s bow shock front to 45 keV and end up with such energies upstream of the bow shock in the foreshock region. In our opinion, being scattered by the oscillations existing in the foreshock, these energetic protons form a population of so-called diffuse ions upstream of the bow shock.

We consider the evolution of the abundance of singly and doubly deuterated isotopologues of water and methanol in a typical protostellar cloud. The main processes determining their abundance in various regions of the cloud have been revealed. We show that the abundances of the deuterated isotopologues can exceed the abundance of the main molecules in a number of cases.

We present the results of our determination of the fundamental atmospheric parameters for the magnetic chemically peculiar Ap stars HD 110066 (AX CVn) and HD 153882 (V451 Her). The determination of the atmospheric parameters ( $$T_{\textrm{eff}}$$ , $$\textrm{log}{g}$$ , $$R/R_{\odot}$$ , and $$L/L_{\odot}$$ ) is based on a self-consistent analysis of high-resolution spectra (ESPaDOnS with $$R=65\,000$$ ) and the observed flux distribution in a wide spectral range. We have derived the radial magnetic field components $$B_{r}=4015\pm 180$$ G for HD 110066 and $$B_{r}=3800\pm 200$$ G for HD 153882 from the observed line splitting and magnetic differential broadening. We have analyzed the chemical composition and the vertical distribution (stratification) of Fe and Cr in atmospheric depth, because the lines of these elements make the greatest contribution to the absorption and stratification affects the distribution of the outgoing flux. We have constructed model atmospheres and determined the evolutionary status of these stars by taking into account the anomalous chemical composition and stratification. The positions of HD 110066 and HD 153882 on the Hertzsprung–Russell diagram confirm the observed decrease in the magnetic field strength with stellar age for stars with masses greater than 2.5 $$M_{\odot}$$ .

We consider the effect of a change in the rate of flux decline in the $$U$$ band for type II-P supernovae (SNe IIP) as a function of metallicity $$Z$$ . Based on this effect, we propose a new method to determine the photometric redshift from the SN IIP light curve in the $$U$$ band. Using the STELLA code, we have constructed model light curves in different bands for different redshifts $$z=0.0$$ , 0.1, and 0.3 as the metallicity in the models decreases from $$Z\sim 10^{-3}$$ to $${\sim}10^{-6}$$ . The flux in the $$U$$ band is shown to reach a plateau at the lowest metallicities. We consider the influence of other parameters as well: the presupernova mass and the mass of radioactive nickel-56.

$$BVI_{c}g^{\prime}r^{\prime}i^{\prime}$$ observations of the classical Cepheid V1467 Cyg have been obtained with the 60-cm telescope at the Caucasus Mountain Observatory of the SAI MSU. Invoking all of the available photometry has allowed an $$O-C$$ diagram spanning a time interval of 107 years to be constructed. This has made it possible to calculate for the first time the rate of evolutionary decrease in the period of V1467 Cyg, $$dP/dt={-}431.5({\pm}{8.2})$$ s yr $${}^{-1}$$ , in agreement with the results of model calculations for the second crossing of the instability strip. The pulsation stability test proposed by Lombard and Koen has confirmed that the evolutionary changes in the period are real.

The algorithm for the reduction of spectroscopic observations with the ASTRONIRCAM camera-spectrograph installed at the 2.5-m telescope of the Sternberg Astronomical Institute of the Moscow State University is described. The instrument allows spectra of astronomical objects with long ( $$280^{\prime\prime}$$ ) and short ( $$10^{\prime\prime}$$ ) slits to be taken in the wavelength range from 1 to 2.5 $$\mu$$ m. The measured resolution reaches $$R=1270$$ for a $$0\overset{{}^{\prime\prime}}{.}9$$ slit; the light efficiency of the system changes from 6 to $$14\%$$ in the working wavelength range in the long-slit mode and from 1 to $$2\%$$ in the cross-dispersion mode. The transmission is limited by the quality of the grisms used. We have estimated the fraction of scattered light and studied the mechanical flexures and temperature drifts of the structure.