We present the results of a new stage of the long-term photometric study of FG Sge which is a quickly evolving central star of the planetary nebula Hen 1-5. Our new observations carried out on the SAI MSU telescopes in the optical ( $$BVR_{C}I_{C}$$ ) and infrared (IR) ( $$JHKLM$$ ) regions in 2008–2021 and 2013–2021, respectively, allowed us to trace the evolution of the star’s brightness in recent years. The most significant observations were performed in 2019 when the star suffered a short clearing of the dust shell and became visible in $$BVR_{C}$$ . Based on the spectral energy distribution of FG Sge in the 0.4–5 $$\mu$$ m wavelength range we derived the dust shell parameters: the size of dust grains $$a=0.01\mu$$ m, the inner radius temperature $$T_{\text{dust}}=900$$ K, optical depth $$\tau(K)=0.5$$ ( $$\tau(V)=4.5$$ ), the total mass of dust $$M_{\text{dust}}=7\times 10^{-5}M_{\odot}$$ . After the short-term clearing of the dust shell in 2019, another dust structure was ejected that resulted in the star fading in all the observed bands. Based on the IR brightness and color curves, we estimated the dust depth growth in 2019–2020.

H 3-75 is a medium-excitation planetary nebula with a binary central star consisting of a hot subdwarf with $$T_{\textrm{hot}}\sim 10^{5}$$ K and a cool giant. We present the results of our photometric and spectroscopic observations obtained in 2020 and analyze data from the literature. The brightness of the cool component of the binary system has been measured in the $$VR_{C}I_{C}JHK$$ bands. We have measured the relative intensities of emission lines in the spectrum of the planetary nebula H 3-75, estimated the extinction, and determined the gaseous envelope parameters. We have found the spectral type and luminosity class of the cool star to be K0 III and analyzed its spectrum. We have estimated the distance to the object ( $$d\sim 3660$$ pc) and the luminosities of the components of the binary system: $$L_{\textrm{cool}}\sim 50L_{\odot}$$ and $$L_{\textrm{hot}}\sim 160L_{\odot}$$ . The hot subdwarf’s parameters $$T_{\textrm{hot}}$$ and $$L_{\textrm{hot}}$$ place the star on the cooling track of post-AGB stars.

We present the results of our new low-resolution spectroscopic observations of the young compact planetary nebula Hb 12 performed in 2011–2020 with SAI MSU telescopes. We have measured the intensities of more than 50 nebular emission lines in the spectral range $$\lambda$$ 3687–9532, detected interstellar absorption features, and conducted a search for absorptions belonging to the possible secondary component of the central star. The extinction coefficient has been estimated from the Balmer decrement to be $$c($$ H $$\beta)=1.15\pm 0.07$$ . The distance has been found by analyzing the interstellar extinction maps to be $$D\approx 2400$$ pc. We have traced the history of the spectroscopic observations of Hb 12, beginning with the first spectra taken by Aller (1951) in 1945. We have detected a systematic increase in the relative intensities of the nebular [O III] $$\lambda$$ 4959 and $$\lambda$$ 5007 lines and a decrease in the relative intensity of the auroral [O III] $$\lambda$$ 4363 line, which has led to an increase in the observed flux ratio $$F(\lambda 4959+\lambda 5007)/F(\lambda 4363)$$ by a factor of $$\sim$$ 4 from 1945 to the present time. The [O III]/[O II] line ratio $$F(\lambda 4363)/F(\lambda 3727+\lambda 3729)$$ remains constant, suggesting that the degree of ionization, on average, for the nebula is invariable. The temperature of the exciting star has been estimated to be $$T\approx 41\,000$$ K. We conclude that a decrease in the electron temperature and, possibly, electron density in the [O III] line formation region is mainly responsible for the spectroscopic variability.

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.

LT Del is a yellow symbiotic system that consists of a bright K3-type giant and a hot subdwarf with a temperature ∼10 5 K. We present the results of our spectroscopic observations of LT Del over the period 2010–2018. In 2017 the star experienced a second low-amplitude (Δ V ∼ 0 ṃ 7) outburst in the history of its studies. The emission spectrum of the star represented in the optical range by hydrogen, neutral and ionized helium lines underwent significant changes in the outburst. The fluxes in the HI and He I emission lines increased by a factor of 5–6, the He II λ4686 line grew by a factor of 10. According to our estimates, in the 2017 outburst the temperature of the exciting star rose to T hot ∼ 130 000 K, while during the first 1994 outburst the change in temperature was insignificant. This suggests cool and hot outbursts of LT Del by analogy with similar events of another yellow symbiotic star, AG Dra.

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 a study of the high-resolution optical spectrum for the hot post-asymptotic giant branch (post-AGB) star, Hen 3-1013, identified as the optical counterpart of the infrared sourceIRAS 14331-6435. For the first time the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9000 \AA\ is carried out. Absorption lines of HI, HeI, CI, NI, OI, NeI, CII, NII, OII, SiII, SII, ArII, FeII, MnII, CrII, TiII, CoII, NiII, SIII, FeIII and SIV were detected. From the absorption lines, we derived heliocentric radial velocities of $V_r=-29.6\pm0.4$ km/s. We have identified emission permitted lines of OI, NI, FeII, MgII, SiII and AlII. The forbidden lines of [NI], [FeII], [CrII] and [NiII] have been identified also. Analysis of [NiII] lines in the gaseous shell gives an estimate for the electron density $N_e\sim10^7$ cm$^{-3}$ and the expansion velocity of the nebula $V_{exp}=12$ km/s. The mean radial velocity as measured from emission features of the envelope is $V_r=-36.0\pm0.4$ km/s. The Balmer lines H$\alpha$, H$\beta$ and H$\gamma$ show P Cyg behaviour which indicate ongoing post-AGB mass-loss. Based on ASAS and ASAS-SN data, we have detected rapid photometric variability in Hen 3-1013 with an amplitude up to 0.2 mag in the V band. The star's low-resolution spectrum underwent no significant changes from 1994 to 2012. Based on archival data, we have traced the photometric history of the star over more than 100 years. No significant changes in the star brightness have been found.

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.