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—The article presents the results of multicolor photometry, medium and low resolution spectroscopy of the red nova V838 Mon remnant for 16 years after the 2002 outburst. We also used the archival photometry with the photographic plates of the Sonneberg and Moscow collections from 1928 to 1994. Analysis of these observational data confirmed that the progenitor of the V838 Mon explosion was a wide pair of B3V type stars of reduced luminosity. A brighter component exploded; it was 36 per cent brighter than its companion, and located on the zero-age main sequence of the Spectrum–Luminosity Diagram. Immediately after the outburst, in the fall of 2002, the remnant was a brown L-type supergiant (sgL), but in the fall of 2003 its spectrum changed to M type with a blue radiation excess appeared in the spectral energy distribution, which we interpreted as the reflection effect of the B type companion on the dust formed on the M star. In 2008, the companion was engulfed by the expanding explosion remnant, a type M supergiant (sgM). When the companion was immersing in the expanding M-star, a void was discovered under the M-star upper layer, in which the companion moved for about 200 days. Over the past 10 years, the luminosity of the M star has increased in the V filter by a factor of 10, and the spectral type has changed from M7.5 to M5.5. Based on radial velocities in the BaII 6497 A and CaI 6572 A lines, a deceleration of the expanding envelope of the M supergiant was detected, and in 2018, the envelope velocity approached to the heliocentric velocity of the star +71 km s−1. Quasi-periodic changes with a period of 320 days appeared then in the light curves, especially clearly expressed in I filter. We assume that the remnant has an elongated structure, and its rotation period is about 640 days. This is probably a gigantic contact system that will become a detached binary system in future development. The observations do not confirm the assumption that the explosion of one of the V838 Mon components was due to the merger of a compact binary system components located in a hierarchical triple one. Two hypotheses were proposed on the nature of the explosion of one of the V838 Mon components, directly based on the early age of this system: (1) the ignition of thermonuclear burning of hydrogen in the core after the gravitational compression of a protostar; (2) the fragmentation of the core inside a rapidly rotating star in the stage of gravitational compression of a protostar, and later, the subsequent defragmentation (merger of the core components) due to the loss of torque.

We present the results of optical timeresolved spectroscopy and photometry of the eclipsing polar HU Aquarii, which were obtained with the help of the scanner of the 6 m telescope in July, 1994 and the CCD photometr of the 1 m telescope in September, 2002. Study of the parameters of Balmer and Hell emission lines in the spectra of HU Aqr has revealed significant variations with the phase of orbital period on a time-scale of 5-10 min and from day to day. Spectral light curves (integrated flux in the range of wavelengths 4000-500oA) have also shown significant variations. The bottom of the primary eclipses in the spectral light curves was 318 :I: 12 s with variable ingress (328-388 s) and egress (85-333 s). The form of the spectral light curves is strongly variable on a time scale of 1-15 hours. In particular the radial velocity curves on July 14, 1994 demonstrated a deviation from a sinusoid near the orbital phase 0.5 which is considered to be the result of occultation of the magnetic part of the accretion stream by the white dwarf. Simultaneously the spectral light curve has shown several features: the dip at the phase 0 is associated with the eclipse of the white dwarf and accretion stream by the secondary; at the phase % 0.85 the pre-eclipse dip due to the eclipse of the accretion spot by the accretion stream; the dip at the phase 0.5 which, as we suppose, is caused by the eclipse of the accretion spot by the white dwarf; at the phase 0.46 the pre-eclipse dip due to the eclipse of the accretion spot by the magnetic part of the stream. The two pre-eclipse dips at the phases 0.85 and 0.46 point to the location of the accretion spot on the magnetic field line between two parts of the accretion stream which come out from the spot along the field line in approximately opposite directions. The direction of the field line and the character of perturbation of the emission lines at the phase 0.5 exclude the location of the accretion spot in the accretion column and is an indication of the hot spot on a magnetic field line where the horizontal or ballistic part of the stream is captured by the magnetic field as it was predicted by Liebert and Stockman, 1985. The duration of the dip at the phase 0.5 gives the upper limit for the distance from the center of the white dwarf to the accretion spot of z 12Rwd. The location of the accretion spot is far from the basis of the accretion column and should be considered as a second hot spot in the system. The hot spot is the principal source of the optical radiation of the system in the high state of accretion. The absence of the dips at the phases 0.5, 0.46 and 0.85 on other neighboring dates of our observations is the evidence of the instability of accretion geometry on the time-scale of 1 day. Photometrical observations during two nights in September 2002 found HU Aqr in the intermediate state, closer to the low state of brightness of the system. The light curve in V filter showed only the primary eclipse without the dips at the phases of 0.5 and 0.85.

We present the results of the study of the red nova PSN J14021678+5426205 based on the observations carried out with the Russian 6-m telescope (BTA) along with other telescopes of SAO RAS and SAI MSU. To investigate the nova progenitor, we used the data from the Digital Sky Survey and amateur photos available on the internet. In the period between April 1993 and July 2014, the brightness of the progenitor gradually increased by 2.2 mag in the V band. At the peak of the first outburst in mid-November of 2014, the star reached an absolute visual magnitude of -12.75 mag but was discovered later, in February 2015, in a repeated outburst at the absolute magnitude of -11.65 mag. The amplitude of the outburst was minimum among the red novae, only 5.6 mag in the V band. The H alpha emission line and the continuum of a cool supergiant with a gradually decreasing surface temperature were observed in the spectra. Such process is typical for red novae, although the object under study showed extreme parameters: maximum luminosity, maximum outburst duration, minimum outburst amplitude, unusual shape of the light curve. This event is interpreted as a massive OB star system components merging accompanied by the formation of a common envelope and then the expansion of this envelope with minimal energy losses.

We study the newly discovered variable star GSC 4560--02157. CCD photometry was performed in 2013--2014, and a spectrum was obtained with the 6-m telescope in June, 2014. GSC 4560--02157 is demonstrated to be a short-period (P=0.265359d) eclipsing variable star. All its flat-bottom primary minima are approximately at the same brightness level, while the star's out-of-eclipse brightness and brightness at secondary minimum varies considerably (by up to 0.6m) from cycle to cycle. Besides, there are short-term (time scale of 0.03-0.04 days) small-amplitude brightness variations out of eclipse. This behavior suggests cataclysmic nature of the star, confirmed with a spectrum taken on June 5, 2014. The spectrum shows numerous emissions of the hydrogen Balmer series, HeI, HeII., 38 pages, 6 figures

We present the multicolor BV RI photometry and BTA/SCORPIO spectroscopy for the red nova V4332 Sgr, performed in 2005–2012. We have analyzed the behavior of the nova remnant, considering our observations along with all the published observations and archival photographic photometry. The atomic and molecular emission spectra show an exponential weakening. Since 2003, the line fluxes have on the average decreased 30-fold. The continuum of the M-type giant in the spectrum has weakened abruptly between 2006 and 2008, twice in the red and by 4 times in the blue range. The variations in the spectral energy distribution of the stellar continuum after the outburst correspond to a decrease in the surface temperature of the M-type giant by 1000 K. The presence of a faint star of about 20m is possible. The star can be either a member of the system or a field star. It is found that the phenomenon of the red nova in this system is not related with the M-type star. The progenitor of the explosion could be a blue straggler, which has disappeared from the spectral energy distribution after the explosion of 1994. It is most likely that this object was a contact binary system. To explain the “cold explosion” of V4332 Sgr, we have accepted the hypothesis of merging components of a contact binary star in a hierarchical triple or multiple system. There are evidences of dynamical destruction of the outburst remnant and accretion of its matter to the M-type giant. The cause of the red nova phenomenon is thought to be a sudden explosive energy release in the center of a star having a massive envelope, and a subsequent expansion of the envelope in the conditions close to adiabatic. As a result of the explosion, thermal energy reaches the surface of the envelope a year or a few years after the outburst, whereas the envelope already has a large surface area. The cause of the explosion in the center of the star can be both a merger event of the nuclei of two stars in the contact system with a formed common massive envelope and instability in the core of a single massive star. Thus, the red novae can be heterogeneous objects at different evolutionary stages.

We report photometric and spectroscopic observations of the optical transient LSQ13zm. Historical data reveal the presence of an eruptive episode (that we label as `2013a') followed by a much brighter outburst (`2013b') three weeks later, that we argue to be the genuine supernova explosion. This sequence of events closely resemble those observed for SN2010mc and (in 2012) SN2009ip. The absolute magnitude reached by LSQ13zm during 2013a ($M_R=-14.87\pm0.25\,\rm{mag}$) is comparable with those of supernova impostors, while that of the 2013b event ($M_R=-18.46\pm0.21\,\rm{mag}$) is consistent with those of interacting supernovae. Our spectra reveal the presence of a dense and structured circumstellar medium, probably produced through numerous pre-supernova mass-loss events. In addition, we find evidence for high-velocity ejecta, with a fraction of gas expelled at more than 20000\kms. The spectra of LSQ13zm show remarkable similarity with those of well-studied core-collapse supernovae. From the analysis of the available photometric and spectroscopic data, we conclude that we first observed the last event of an eruptive sequence from a massive star, likely a Luminous Blue Variable, which a short time later exploded as a core-collapse supernova. The detailed analysis of archival images suggest that the host galaxy is a star-forming Blue Dwarf Compact Galaxy., Comment: Accepted for publication on MNRAS

V934 Her is a detached system, consisting of a cool giant and a neutron star. The neutron star accretes matter fromthe stellarwind of the giant.Multicolor photoelectric observationsmade in 1997–2011 revealed a 415-day period, close to that, discovered spectroscopically from radial velocities. This period is considered to be the orbital period of orbital motion of the neutron star around the red giant. The wave with this period in the U-band has the largest amplitude 0m.12. We also detected multiperiodic pulsations of the red giant. The light curve in the V -band is dominated by a pulsation wave with the period of 28.82 days and the amplitude of 0m.10.

We present photometry and moderate-resolution spectroscopy of the luminous red variable [HBS2006] 40671 originally detected as a possible nova in the galaxy M33. We found that the star is a pulsating Mira-type variable with a long period of 665 days and an amplitude exceeding 7 mag in the R band. [HBS2006] 40671 is the first confirmed Mira-type star in M33. It is one of the most luminous Mira-type variables. In the K band its mean absolute magnitude is M_K = -9.5, its bolometric magnitude measured in the maximum light is also extreme, M_bol = -7.4. The spectral type of the star in the maximum is M2e - M3e. The heliocentric radial velocity of the star is -475 km/s. There is a big negative excess (-210~km/s) in radial velocity of [HBS2006] 40671 relative to the average radial velocity of stars in its neighborhood pointing at an exceptional peculiar motion of the star. All the extreme properties of the new Mira star make it important for further studies.

CI Cam is a well-known B[e]-type star and an X-ray transient, which underwent a powerful outburst in all parts of electromagnetic spectrum in 1998. An extensive V-band photometric monitoring revealed that rapid variability of CI Cam on the time scale of several hours is due to the pulsations of the B4 III–V type component. High-precision CCD photometry of the star was performed with the 50-cm Maksutov (meniscus) telescope of the Crimean station of the Sternberg Astronomical Institute of Moscow State University in December 2006 during the quiescent state of the star. The data are analyzed using the methods of discrete Fourier transform and by modeling of the light curve. The amplitude spectrum of the star is dominated by two peaks corresponding to the periods of 0.41521 and 0.26647 days with the period ratio close to 3:2. At least two pulsation modes are excited and the star exhibits signs of the resonance between modes.

Our long-time monitoring of the B[e] star and transient X-ray source CI Cam during quiescence following the 1998 outburst demonstrates that the complex, stratified circumstellar envelope has tended to stabilize after this structure was perturbed by the passage of a shock wave from the outburst. The star’s U BV R brightness shows slow, possibly cyclic, variations with an amplitude of about 0.2m. We determined the spectral type of the primary, B4III-V, based on the widths of the absorption wings of high-numbered Balmer lines. A Doppler shift of 460 km/s was detected for the Hell λ4686 A emission line. The shifts in this line yield an orbital period of 19.41 days, which is also manifested itself in the photometric data as a wave with a V amplitude of 0.034m. The orbit is elliptical, with an eccentricity of 0.62. It is most likely that the secondary is a white dwarf surrounded by an accretion disk. The primary’s mass exceeds 12 M ⊙ . The system may be at a late stage of its evolution, after the stage of mass exchange.

Results of simultaneous INTEGRAL and optical observations of galactic microquasar SS433 in May 2003 and INTEGRAL/RXTE observations in March 2004 are presented. Persistent precessional variability with a maximum to minimum uneclipsed hard X-ray flux ratio of \sim 4 is discovered. The 18-60 keV X-ray eclipse is found to be in phase with optical and near infrared eclipses. The orbital eclipse observed by INTEGRAL in May 2003 is at least two times deeper and apparently wider than in soft X-ray band. The broadband X-ray spectrum 2-100 keV simultaneously detected by RXTE/INTEGRAL in March 2004 can be described by bremsstrahlung emission from optically thin thermal plasma with kT\sim 30 keV. The optical spectroscopy with the 6-m SAO BTA telescope confirmed the optical companion to be an A5-A7 supergiant. For the first time, spectorscopic indications of a strong heating effect in the optical star atmosphere are found. The measurements of absorption lines which are presumably formed in the non-illuminated side of the supergiant yield its radial velocity semi-amplitude K_v=132\pm 9 km/s. The analysis of the observed hard X-ray light curve and the eclipse duration, combined with spectroscopically found optical star radial velocity corrected for the strong heating effect, allows us to model SS433 as a massive X-ray binary. Assuming that the hard X-ray source in SS433 is eclipsed by the donor star that exactly fills its Roche lobe, the masses of the optical and compact components in SS433 are suggested to be M_v\approx 30 M_\odot and M_x\approx 9M_\odot, respectively. This provides further evidence that SS433 is a massive binary system with supercritical accretion onto a black hole., 15 pages, AA style, 17 figures, accepted for publication in AA; replaced to match the accepted version

We present the results of our CCD photometric and moderate-dispersion spectroscopic observations of the binary system V4641 Sgr, which contains a black hole of mass ≈9.5M⊙ and a normal B9III star. The photometric light curve reveals an ellipticity effect with very high amplitudes in V and R, 0.40m and 0.37m, and the color curve shows that the surface temperature is nonuniform. All this testifies to tidal distortion of the normal star's surface due to the massive companion and to a high inclination of the orbit to the line of sight. In June and July 2002, during quiescence, we obtained data during three flares with amplitudes up to 0.26m. In particular, spectroscopic observations were acquired near the time of the black hole's inferior conjunction. One hour before conjunction, a depression by EW=0.5 A was observed in the red wing of the Hα absorption line, interpreted as absorption by gas flowing in the direction from the observer toward the normal star. This flow is apparently associated with a rarefied gas disk around the black hole, and the conjunction grazes the stellar surface if the orbital inclination is close to 70.7°. The maximum velocity along a circular Keplerian orbit is 650 km/s at a distance of R=0.15–0.20a from the black hole (where a is the component separation). Thus, we find the mass of the black hole to be MBH=7.1–9.5M⊙, confirming the model of Orosz et al. (2001).

The photometric monitoring of Nova V 1494 Aql at the Sternberg Astronomical Institute and at the Special Astrophysical Observatory in July and September 2002 has confirmed that the star is an eclipsing system. Its orbital period was improved, $$0\mathop .\limits^d 1346141(5)$$ . Light elements and eight mid-eclipse times are given. The light-curve shape suggests that the star is an intermediate polar.

We compare U BV R photometry of CI Cam obtained using facilities at the Sternberg Astronomical Institute in 1998–2001 and medium-resolution spectroscopy obtained at the Special Astrophysical Observatory in the same period. The photometric measurements and fluxes in the Balmer and some Fe II emission lines in the post-outburst quiescent spectrum of CI Cam display cyclic variations with a period of 1100±50d. These variations could be due to orbital motion in a widely separated binary; one component, a giant, produces a reflection effect on the surface facing the other, compact component. Our V-band photometry confirms the $$11\mathop d\limits_. 7$$ period found by Miroshnichenko before the outburst but shows a lower amplitude (3%). We investigate the possibility that this photometric period is the same as the rotational period of the radio jets measured using the VLA. Based on modeling of the radio map, we estimate the inclination of the jet rotation axis to the line of sight, i=35°–40°; the angle between the rotational axis and jet ejection direction, ϑ=7°–10°; and the jet spatial velocity, 0.23–0.26 c. The equivalent widths and fluxes of various spectral lines show different variation amplitudes during outburst and significantly different behavior during quiescence. We have detected five patterns of behavior, which provide evidence for stratification of the gas and dust envelope surrounding the system. The brightening of the [N II] forbidden line is delayed by 50–250d relative to the X-ray outburst maximum.

We report the first results of a systematic search near the plane of the Galaxy for the so called very active stars (VAS), which are characterized by a hard X-ray spectrum and activity in the radio domain. Candidates with hard X-ray binary-like spectra have been selected from the Bright ROSAT Source Catalogue in the Zone of Avoidance ($| b | < 20{^o}$) and were tentatively identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the ATCA and VLA. Precise radio coordinates have led to unambiguous optical identification for 60 candidates, and a sub-sample of five of themhas been observed with the VLT. Also some discovery and confirmatory spectra were obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate dispersion, made with the FORS1 spectrograph of the VLT has revealed two stellar objects (one of them, VASC J1628-41, is definitivelya binary VAS), one new AGN and two featureless spectrum sources. One of these objects, VASC J1353-66, shows a marginal evidence of proper motion, which, if confirmed, would imply the discovery of a new type of galactic source.

We present the results of our photometric and spectroscopic follow-up of the intermediate-luminosity optical transient AT 2017jfs. At peak, the object reaches an absolute magnitude of Mg = -15:46 ± 0:15 mag and a bolometric luminosity of 5:5 × 1041 erg s-1. Its light curve has the doublepeak shape typical of luminous red novae (LRNe), with a narrow first peak bright in the blue bands, while the second peak is longer-lasting and more luminous in the red and near-infrared (NIR) bands. During the first peak, the spectrum shows a blue continuum with narrow emission lines of H and Fe II. During the second peak, the spectrum becomes cooler, resembling that of a K-type star, and the emission lines are replaced by a forest of narrow lines in absorption. About 5 months later, while the optical light curves are characterized by a fast linear decline, the NIR ones show a moderate rebrightening, observed until the transient disappears in solar conjunction. At these late epochs, the spectrum becomes reminiscent of that of M-type stars, with prominent molecular absorption bands. The late-time properties suggest the formation of some dust in the expanding common envelope or an IR echo from foreground pre-existing dust. We propose that the object is a common-envelope transient, possibly the outcome of a merging event in a massive binary, similar to NGC4490-2011OT1. © ESO 2019., We thank Rubina Kotak for useful suggestions. YZC is supported by the China Scholarship Council (No. 201606040170). MF is supported by a Royal Society - Science Foundation Ireland University Research Fellowship. NER acknowledges support from the Spanish MICINN grant ESP2017-82674-R and FEDER funds. S.Bose, PC and SD acknowledge Project 11573003 supported by NSFC. This research uses data obtained through the Telescope Access Program (TAP), which has been funded by the National Astronomical Observatories of China, the Chinese Academy of Sciences, and the Special Fund for Astronomy from the Ministry of Finance. S. Benetti is partially supported by PRIN-INAF 2017 >Toward the SKA and CTA era: discovery, localization, and physics of transient sources.> (PI: M. Giroletti). KM acknowledges support from STFC (ST/M005348/1) and from H2020 through an ERC Starting Grant (758638). AF acknowledges the support of an ESO Studentship. AMT acknowledges the support from the Program of development of M.V. Lomonosov Moscow State University (Leading Scientific School >Physics of stars, relativistic objects and galaxies>. CT, AdUP, DAK and LI acknowledge support from the Spanish research project AYA2017-89384-P, and from the >Center of Excellence Severo Ochoa> award for the IAA (SEV-2017-0709). CT and AdUP acknowledge support from funding associated to Ramon y Cajal fellowships (RyC-2012-09984 and RyC-2012-09975). DAK and LI acknowledge support from funding associated to Juan de la Cierva Incorporacion fellowships (IJCI-2015-26153 and IJCI-2016-30940). The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, STScI, NASA under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the US NSF under Grant No. AST-1238877, the University of Maryland, and Eotvos Lorand University (ELTE). Operation of the Pan-STARRS1 telescope is supported by NASA under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the NEO Observation Program. This paper is also based upon work supported by AURA through the National Science Foundation under AURA Cooperative Agreement AST 0132798 as amended. ATLAS observations were supported by NASA grant NN12AR55G. NUTS is supported in part by the Instrument Center for Danish Astrophysics (IDA). This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 199. D-0143(G,I,K,L). This work makes use of observations from the LCOGT network. It is also based on observations made with the 2.2m MPG telescope at the La Silla Observatory, the Nordic Optical Telescope (NOT), operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias; the 1.82m Copernico Telescope of INAF-Asiago Observatory; the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias, in the Island of La Palma; the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council; the 6m Big Telescope Alt-azimuth and the Zeiss-1000 Telescope of the Special Astrophysical Observatory, Russian Academy of Sciences. We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University and NSF grant AST-1515927. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CAS-SACA), the Villum Foundation, and George Skestos.