We present the results of a UBVRcIc and JHKLM photometric monitoring campaign conducted in 2016–2020, low-resolution spectroscopy, and an analysis of the ASAS-3 and ASAS-SN photometric data for the semiregular variable and post-AGB candidate AU Vul (IRAS 20160+2734). The star is shown to experience quasi-periodic brightness oscillations with a variable amplitude and periods of 67–75 and 145–150 days in different time intervals. Its spectrum at maximum light is classified as that of an early G supergiant. In this spectrum H $$\alpha$$ , H $$\beta$$ , and higher Balmer lines are weakened, possibly, because of emission components, while the absorptions of s-process elements (Ba II, Sr II, and Y II) are slightly strengthened. An H $$\alpha$$ emission has been detected in the spectrum on the ascending branch of the light curve. TiO absorption bands are observed in the spectra at different brightness levels with maximum intensity at minimum light. The spectral energy distribution in the range 0.44 ( $$B$$ )–2.2 ( $$K$$ ) $$\mu$$ m corresponds to spectral types from G2I at maximum to G8I at minimum light. An emission excess has been detected in the $$L$$ and $$M$$ bands. The spectral energy distribution in the range 0.44–90 $$\mu$$ m constructed from our own observations and WISE, MSX, IRAS, and AKARI data is satisfactorily described by the sum of three components: a star and dust envelopes with $$T_{\textrm{hot}}=1000$$ K and $$T_{\textrm{cold}}=150$$ K. Using the distance based on the Gaia EDR3 parallax, $$D\approx 2300$$ pc, we have estimated the absolute magnitude, $$M_{V}\approx-4\overset{m}{.}45$$ , and the luminosity, $$L\approx 5450L_{\odot}$$ . A comparison with evolutionary models has shown that AU Vul may be at the very beginning of the post-AGB stage of evolution and have a mass $${\sim}0.55M_{\odot}$$ . We note that the star has a number of properties that distinguish it from typical post-AGB objects and make it akin to RV Tau stars.

$$UBVJHKLM$$ photometry for the carbon Mira star V CrB are presented. The infrared observations were carried out in the time interval 1989–2018, while the $$U$$, $$B$$, and $$V$$ data were obtained in 2001–2014. The light and color curves are analyzed. The pulsation period of V CrB has been found to be $$355\overset{\textrm{d}}{.}2$$ in the infrared $$JHKLM$$ bands and $$352^{\textrm{d}}$$ for the optical $$BV$$ band. In the $$JHK$$ bands, apart from periodic pulsations, there are distinct sinusoidal variations in the average brightness level with a characteristic period of $${\sim}8300$$ days. Color–magnitude relationships have been revealed for the infrared and optical bands. The phase curves exhibit the wavelength dependence of the brightness variability amplitude. The light curves for various bands and colors are discussed. We have constructed the model of a spherically symmetric circumstellar dust envelope that allows the observed spectral energy distribution at both maximum and minimum light to be reproduced equally well (within the model assumptions) and is consistent with the observations of V CrB by differential speckle polarimetry. The model is characterized by the following parameters: the optical depth is $$\tau_{K}=0.33$$, the inner and outer radii of the envelope are 8 and 40 000 AU, respectively. The envelope contains spherical carbon dust grains ($$3/4$$ by mass) and silicon carbide dust grains. Dust grains with a radius of 0.5 $$\mu$$m account for $$90\%$$ of the envelope mass. The remaining $$10\%$$ of the mass is accounted for by finer dust with a grain radius of 0.1 $$\mu$$m. Based on the observational data, we have estimated the bolometric flux from V CrB: $$2.6\times 10^{-7}$$ and $$5.1\times 10^{-7}$$ erg cm$${}^{-2}$$ s$${}^{-1}$$ at minimum and maximum light, respectively. The effective temperature of the star is $$T_{\textrm{max}}=3000$$ K at maximum light and $$T_{\textrm{min}}=2400$$ K at minimum light.

We present the results of a new epoch (2009–2019) of a long-term (50 years) photometric monitoring of the variable planetary nebula IC 4997 (QV Sge). The integral (star + nebula) $$UBV$$ light curves display a continuing brightening of $$0\overset{m}{.}15$$ in $$V$$ , a slight rise ( $${<}0\overset{m}{.}1$$ ) in $$B$$ , and constancy in $$U$$ . The $$B-V$$ color has got redder from $$0\overset{m}{.}4$$ in 2000 to $$0\overset{m}{.}7$$ in 2019, whereas the $$U-B$$ color has not changed significantly at that time. We carried out near infrared (IR) $$JHKL$$ photometry in 2019, and comparing it to the data obtained in 1999–2006 we found the source to be fainter by $$0\overset{m}{.}4$$ in $$L$$ and bluer by $$0\overset{m}{.}2$$ in the $$K-L$$ color. The long-term brightness variations in the optical and IR regions are shown to be due mostly to the changing input of emission lines to the integral light. Low-dispersion spectroscopic observations carried out in 2010–2019 revealed a continuing decrease in the [O III] $$\lambda$$ 4363 to H $$\gamma$$ intensity ratio: it decreased by a factor of $${\sim}3$$ in 30 years and reached the level of 1960–1970. We discovered that the absolute intensities of [O III] $$\lambda$$ 4959 and $$\lambda$$ 5007 nebular lines had increased by a factor of $${>}2$$ from 1990 to 2019, whereas the [O III] $$\lambda$$ 4363 auroral line had weakened by a factor of 2 comparing to the maximum value observed in 2000. The variation of H $$\beta$$ absolute intensity in 1960–2019 was shown to be similar to that of [O III] $$\lambda$$ 4959 (and $$\lambda$$ 5007), but of smaller amplitude. The electron density in the outer part of the nebula was estimated from the [S II] and [Cl III] lines. Basing on the data on absolute intensities for the H $$\beta$$ , [O III] $$\lambda 4363,4959$$ lines and their ratios we propose a possible scenario describing the change of physical conditions ( $$N_{e}$$ , $$T_{e}$$ ) in IC 4997 in 1970–2019. The main features of spectral variability of IC 4997 could be explained by a variation of electron temperature in the nebula caused by not so much the change in ionizing flux from the central star as the variable stellar wind and related processes. The photometric and spectral changes observed for IC 4997 in 1960–2019 may be interpreted as an observable consequence of a single episode of enhanced mass loss from the variable central star.

We present the results of our photoelectric UBV observations of the yellow symbiotic star LT Del over 2010–2018. The binary system LT Del, which consists of a bright K giant and a compact hot star with a temperature of ~100 000 K, has an orbital period of 476 days. In 2017 the variable experienced a second low-amplitude (Δ V ~ 0 ṃ 7) outburst in the history of its studies whose maximum occurred at an orbital phase of 0.15±0.05. The outburst duration was ~60 days. The B–V and U–B colors in the outburst became noticeably bluer. A difference in the photometric behavior of the star in the 1994 and 2017 outburst has been detected. In the orbital cycle preceding the 2017 outburst a secondary minimum with a depth of 0 ṃ 7 and 0 ṃ 20 appeared in the B and V light curves, respectively, whose cause is discussed. The phase light and color curves are presented and explained; the position of the star on the color-color diagram is interpreted. We have estimated the parameters of the cool and hot components of the system based on the distance determination from Gaia DR2.

We present the results of multicolor (UBV JHKLM) photometry (2009–2017) and low-resolution spectroscopy (2016–2017) of the semi-regular variable V1427 Aql = HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude (ΔV<0 . m 2) semi-periodic brightness variations superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations; brightness oscillations with no significant change of color are also observed. The UBV data for the 2009–2011 interval are well reproduced by a superposition of two periodic components with P = 170d and 141d (or P = 217d—the one year alias of P = 141d). The variation became less regular after 2011, the timescale increased and exceeded 250d. Unusual photometric behavior was seen in 2015 when the star brightness increased by 0 . m 25 in the V filter in 130 days and reached the maximum value ever observed in the course of our monitoring since 1990. In 2009–2016 the annual average brightness monotonically increased in V, J, K, whereas it decreased in U and B. The annual average U − B, B − V, and J − K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008–2016 interval, the radius increased by ~24%, and the luminosity grew by ~19%. We review the change of annual average photometric data for almost 30 years of observations. Low-resolution spectra in the λ4000−9000 Å wavelength range obtained in 2016–2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994–2008 interval, i.e., the Ba II and near-infraredCa II triplet absorptions have gotten stronger while the OI λ7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.

We present the results of multicolor (UBV JHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude (∆V < 0.2) semi-periodic brightness variation superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations, and brightness oscillations with no significant change of color are also observed. The UBV data for the 2009-2011 interval are well reproduced by a superposition of two periodic components with P = 170 and P = 141 (or P = 217 – the one year alias of P = 141). The variation became less regular after 2011, the timescale increased and exceeded 250. An usual photometric behavior was seen in 2015 when the star brightness increased by 0.25 in the V filter in 130 days and reached the maximum value ever observed in the course of our monitoring since 1990. In 2009-2016 the annual average brightness monotonically increased in V , J , K, whereas it decreased in U and B. The annual average U − B, B − V , and J −K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ∼400 K in the 2008-2016 interval, the radius increased by ∼24 %, and the luminosity grew by ∼19 %. We review the change of annual average photometric data for almost 30 years of observations. Low-resolution spectra in the λ4000 − 9000 Å wavelength range obtained in 2016-2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994-2008 interval, i.e. the BaII and near-infrared CaII triplet absorptions have turned stronger while the OI λ7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.

We present the results of our photometric UBV JHKL observations for the symbiotic star V1413 Aql obtained in 2012–2018. An analysis of the data has shown that inMay 2017 the system passed to a quiescent state with B − V ≈ 0ṃ 6 for the first time since 1993. It lasted no more than five months. The J − K color at the primary minimum of 2012 reached 1ṃ 5, which, given the interstellar reddening, corresponds to spectral type M5-M6 III of the cool component. A secondary minimum has been detected at φ ≈ 0.5 on the JK phase light curves constructed for the dates of observations with B ≥ 13.

The U BV photometry and low-resolution spectroscopy for the semiregular variable AI CMi, a candidate for post-AGB objects, performed in 1996–2016 and 2000–2013, respectively, are presented. The star showed multiperiodic brightness variations with an amplitude up to \(1\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5\) in the V band, a significant (up to \(0\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 4\)) bluing of the B − V and U − B colors as the star faded, and a change of its spectrum from G5 I to K3–5 I, depending on its brightness. A possible long-term fading of AI CMi below \(8\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{m} 5\) in the period from May 2013 to early 2015 is observed in the light curve. The colors in this episode did not change the pattern of their unusual behavior with brightness. The main feature of the spectrum for AI CMi is the appearance and strengthening of TiO absorption bands as its brightness declines, which are atypical in the spectra of ordinary G5–K3 supergiants. The bluing of the B − V and U − B colors is interpreted as the blanketing of stellar radiation predominantly in V (and to a lesser extent in B) by the TiO absorption bands whose intensity increases dramatically with decreasing brightness. Another cause of the bluing can be the scattering of stellar radiation by small dust particles in the gas–dust shell of AI CMi. The star’s continuum-normalized spectra over the period from 2000 to 2013 in the wavelength range 4200 to 7700 or 9200 Å are presented. These were taken at different phases of the pulsation cycle and clearly demonstrate the behavior of the TiO absorption bands depending on the V magnitude and B − V color. The equivalent widths of individual TiO bands weremeasured, and their correlation with the photometric parameters of the star is shown. AI CMi belongs to the O-rich branch of AGB/post-AGB supergiants and has a luminosity of ~4000 L ⊙ at a distance of 1500 ± 700 pc. The mass of AI CMi is most likely small and close to the lower mass limit for post-AGB stars. The connection of the star’s pulsational activity and nonstationary wind with the formation of its molecular and dust shells is discussed briefly.

The results of long-term photometric and spectroscopic observations of the young compact planetary nebula Vy 2-2 (PNG 045.4-02.7) are presented. The UBV photometry in 1990–2016 has revealed a slight brightness trend in the yearly averaged data, most pronounced in the V band. We have measured the relative fluxes of optical emission lines on the spectrograms taken with the 1.25-m telescope at the Southern Station of the SAI MSU in 1999–2016, estimated the absolute flux in the Hβ line to be F(Hβ) = (2.1 ± 0.4) × 10−12 erg cm−2 s−1, and determined the interstellar extinction constant c(Hβ) = 1.8. The electron temperature and density in the nebula have been estimated from diagnostic line ratios: Te = (10−12) × 103 K and Ne ≥ 105 cm−3. To detect any possible evolutionary changes, we have compared the new observations with the archival data obtained over the entire history of spectroscopic observations of Vy 2-2. No significant changes in the relative intensities of the strongest emission lines and the integrated flux in the Hβ line exceeding the observational errors have been found. We have revealed a tendency for the intensity ratio F(λ4363)/F(λ4959) to decrease with time, which may be related to a decrease in the electron density in the nebula. Based on our photometric and spectroscopic data, we have estimated the luminosity of the central star of Vy 2-2, which corresponds to the evolutionary tracks for the most massive post-AGB stars of the O-rich sequence.

Abstract We present the results of our long-term UBV JHKLM photometry and spectroscopic monitoring of the symbiotic nova V1016 Cyg. After its outburst in 1964, the star showed fading in the U, B, V bands at a rate of about 0.03 mag per year. The behavior of the B − V and U − B color indices reflects variations of the emission lines, fading of the erupted component, weakening and reddening of the cool giant. Also, monotonic color and brightness variations in the infrared (IR) were observed at a scale of several thousand days. After 2004, the yearly mean IR brightness showed a decline and IR colors, reddening, due to the increase of the optical depth of the dust. The parameters of the cool star and of the dust envelope were estimated. The pulsation period of the Mira-type variable was refined, P = 465±5 days. The Mira’s photospheric temperature varied from 2100 to 2700 K in the pulsation cycle. The mass of the dust shell has grown twice during the recent decade, at a dust penetration rate of ∆M dust ~ 10−7 M ⊙/yr. Our spectroscopic monitoring of V1016 Cyg over 1995−2013 showed variations in the emission line strengths. The absolute fluxes of most lines decreased after 2000, whereas the relative intensities of [O III], [Ar III], [Fe VII], [Ca VII] lines with respect to Hβ are increasing after the possible minimum that could happen in the 1990s. An essential flux decline (approximately ten-fold between 1995 and 2013) in the Raman scattered O VI line at λ6825 shows the change of conditions in its formation zone, due to absorption of O VI 1032 Å quanta in the new dust shell of the cool component.