We analyze spectroscopic and photometric observations of the eclipsing polar BS Tri. The polar's light curve shape variations can be interpreted by changing contributions of the accretion stream to the integral radiation of the system. Based on the radial velocity curves of the irradiated part of the secondary, we refine the masses of the system components, $M_1 = 0.60 \pm 0.04 M_{\odot}$, $M_2 \approx 0.12 M_{\odot}$, and the orbital inclination, $i=85\pm 0.5^{\circ}$. The polar's spectra reveal cyclotron harmonics forming in an accretion spot with a magnetic field strength of $B=22.7 \pm 0.4$ MG and an average temperature of $T \sim 10$ keV. In addition to the cyclotron harmonics, the BS Tri spectra contain Zeeman components of H$\alpha$ line, which are probably formed in the cool halo near the accretion spot. The orientation of the magnetic dipole and the coordinates of the accretion spot are estimated by modeling the light curves of the polar. We show that for a satisfactory description of the BS~Tri light curves we have to take into account the variability of the spot's optical depth along the line of sight. Doppler maps of BS Tri show a part of the accretion stream with a trajectory close to ballistic near the Lagrange point L$_1$, and another part of the stream moving along the magnetic field lines. The estimate of the stagnation region position found from the Doppler tomograms is consistent with the photometric estimates of the accretion spot position.

The article considers the poorly studied states of hydrogen gas (called here “quasi-resonant”) with anomalously high intensity ratios of the lines in the Balmer series, $$I({{{\text{H}}}_{\alpha }}){\text{/}}I({{{\text{H}}}_{\beta }})$$. It is shown that such states arise in the case of large optical thicknesses of the emitting gas occurring at the frequencies of the Balmer lines, when the gas tends to a thermalization state. The influence on the formation of such states of basic model parameters as temperature, atomic density, and the characteristic size of the emitting region, as well as the presence of external radiation, is studied on simple models. It is shown that quasi-resonant states can exist in the magnetospheres of T Tauri stars at moderate accretion rates.

The formation of hydrogen emission lines in the magnetospheres of young stars is considered. The magnetosphere is assumed to be formed by a dipolar magnetic field whose axis is aligned with the stellar rotation axis. The radiative transfer in spectral lines is considered in the Sobolev approximation by taking into account the nonlocal radiative coupling. The gas density and temperature distributions in the magnetosphere are taken to be the same as those in Hartmann et al. (1994). The results of our calculations of the H α and H β line intensities and profiles are presented for the case of a slowly rotating star. We separately consider the magnetospheric models in which the infalling gas is heated by radiation from an accretion spot on the stellar surface and the case where the axis of the magnetosphere is tilted with respect to the stellar rotation axis. Rotational modulation of the spectral line profiles with the stellar rotation period is observed in the latter case.

Optical, J and K photometric observations of the KV UMa black hole X-ray nova in its quiescent state obtained in 2017-2018 are presented. A significant flickering within light curves was not detected, although the average brightness of the system faded by approximate to 0.1(m) over 350 d. Changes in the average brightness were not accompanied with an increase or decrease in the flickering. From the modelling of five light curves the inclination of the KV UMa orbit and the black hole mass were obtained: i = 74 +/- 4 degrees, M-BH = (7.06-7.24) M-circle dot, depending on the mass ratio used. The non-stellar component of the spectrum in the range lambda = 6400-22 000 angstrom can be fitted by a power law F-lambda similar to lambda(alpha), alpha approximate to -1.8. The accretion disc orientation angle changed from one epoch to another. The model with spots on the star was inadequate. Evolutionary calculations using the SCENARIO MACHINE code were performed for low-mass X-ray binaries, with a recently discovered anomalously rapid decrease of the orbital period taken into account. We show that the observed decrease can be consistent with the magnetic stellar wind of the optical companion, whose magnetic field was increased during the common-envelope stage. Several constraints on evolutionary scenario parameters were developed.

The transient event labelled as TCP J05074264+2447555 recently discovered towards the Taurus region was quickly recognized to be an ongoing microlensing event on a source located at distance of only 700-800 pc from Earth. Here, we show that observations with high sampling rate close to the time of maximum magnification revealed features that imply the presence of a binary lens system with very low-mass ratio components. We present a complete description of the binary lens system, which host an Earth-like planet with most likely mass of 9.2 +/- 6.6 M-circle plus. Furthermore, the source estimated location and detailed Monte Carlo simulations allowed us to classify the event as due to the closest lens system, being at a distance of similar or equal to 380 pc and mass similar or equal to 0.25 M-circle dot.

In 2015-2016, the X-ray nova A0620-00 in quiescence was observed in the passive state, according to the terminology of Cantrell et al., and in less than 230 d it transited into the active state. The system mean luminosity increased by 0.20 mag in the optical and by 0.25 and 0.30 mag in J and K infrared bands, respectively, while the orbital light curves changed drastically, and the flickering amplitude more than doubled, The mean light-curve analysis performed in the context of two models argued that the interaction region where the gas stream reaches the disc is responsible for these effects. The growth of this region's luminosity in the active state implies the increase of the mass transfer rate via the L1 point, likely due to non-stationary processes in the donor star atmosphere. The non-stellar spectrum F-lambda in the observed range lambda lambda 6400-22 000 angstrom obeys a power law of lambda(alpha) where alpha = -(2.13 +/- 0.1) in the passive stale and alpha = -(1.85 +/- 0.1) in the active, while the absolute mean square flickering amplitude behaves like Delta F-fl(lambda) similar to lambda(-2.36) in both states of activity.

An analysis of the new \(U,B,V,R_{C},I_{C}\)-photometry of the cataclysmic variable RZ LMi obtained in 2016–17 showed the largest \((U-B)\) colour excess in quiescence as well as during the decline of brightness, associated with the outbursts activity. The smallest \((U-B)\) colour excess was found during the brightness increase from the quiescence. In contrast to the \((U-B)\) colour index, the \((B-V),(V-R_{C}),(R_{C}-I_{C})\) colour indices exhibits the largest colour excesses near the maximum of the outburst and the smallest during the quiescence. The \((B-V)\) colour index showed also a large excess 1–2 days before a minimum. The detailed study of superhumps during the maximum of activity reveals the largest \((U-B)\) colour excess at the time of the minimum brightness of superhumps. The \((B-R_{C})\) colour index exhibits a similar behaviour, but with a phase shift of \(+0.1\mbox{--}{+}0.2\) period of superhumps. The tracks in two-colour and colour-magnitude diagrams during superoutbursts are compared with the data for other cataclysmic variables during their outbursts as well as with published theoretical calculations.

We have monitored the return to quiescence of novae previously observed in outburst as supersoft X-ray sources, with optical photometry of the intermediate polar (IP) V4743 Sgr and candidate IP V2491 Cyg, and optical spectroscopy of these two and seven other systems. Our sample includes classical and recurrent novae, short period (few hours), intermediate period (1-2 days) and long period (symbiotic) binaries. The light curves of V4743 Sgr and V2491 Cyg present clear periodic modulations. For V4743 Sgr, the modulation occurs with the beat of the rotational and orbital periods. If the period measured for V2491 Cyg is also the beat of these two periods, the orbital one should be almost 17 hours. The recurrent nova T Pyx already shows fragmentation of the nebular shell less than 3 years after the outburst. While this nova still had strong [OIII] at this post-outburst epoch, these lines had already faded after 3 to 7 years in all the others. We did not find any difference in the ratio of equivalent widths of high ionization/excitation lines to that of the Hbeta line in novae with short and long orbital period, indicating that irradiation does not trigger high mass transfer rate from secondaries with small orbital separation. An important difference between the spectra of RS Oph and V3890 Sgr and those of many symbiotic persistent supersoft sources is the absence of forbidden coronal lines. With the X-rays turn-off, we interpret this as an indication that mass transfer in symbiotics recurrent novae is intermittent.

Outburst of the star CSS160603: 162117+441254 in June 2016 attracted an attention of astronomers. Two years before outburst the star was classified as the contact system of the W UMa-type having a period of P-orb = 0.(d)2079. Outburst was lasting for several days, then light curve and brightness of J1621 returned to the initial form.

Photometric observations of the low-mass X-ray binary system A0620-00=V616 Mon are performed in the optical (unfiltered light, lambda_eff~6400A) and the near-infrared J and K-bands. The mean system flux, the orbital light curve shape and the flickering amplitude dependences on wavelength are examined for two activity stages of the system remaining in quiescence. In 2015-16 A0620-00 was in passive stage (as by Cantrell et al., 2008) exhibiting the regular orbital light curves and low flickering. In less than 230 days in 2016-17 the system switched into active stage: the brightness increased by ~0.2-0.3 mag, the orbital light curve changed while the flickering amplitude increased more than twice. The object regular orbital light curves were fitted by models with "cold" spots on the optical star surface and without those. These models reproduce the observed orbital light curves both in passive and in active stages. The dependence of the mean square flickering amplitude (in fluxes, extinction corrected) on wavelength is computed in the lambda 6400-22000AA range. In active stage, the observed flickering amplitude decreases over the whole studied range and may be represented as Delta F_fl~lambda^{-2} which corresponds to the free-free emission of optically thin high-temperature plasma. In passive stage, flickering obeys Delta F_fl~lambda^{-4} law in the range 6400-12500AA that corresponds to the thermal radiation of optically thick high-temperature plasma. At longer wavelengths the flickering amplitude dependence is flat which may imply existence of a synchrotron component of the relativistic jets emission. These flickering features let us propose that the flickering mechanism includes at least two components: thermal and, apparently, synchrotron, that agrees with the recent discovery of the variable linear polarization of the IR system emission (Russell et al., 2016).

Multifrequency observations of the classical CO nova V339 Del, detected in all frequencies from γ-rays to radio, are reviewed. Nova V339 Del was discovered on 2013 August 14.584 UT. The times of decline from the brightness maximum V = 4.4 mag, reached on 2013 August 16.47, UT, was estimated as t2,V = 10 days, t3,V = 18 days, so V339 Del can be classified as a fast nova with super-Eddington luminosity at maximum. The maximum-magnitude-rate-of-decline relations were used to determine an absolute magnitude at maximum MV,max = −8.70± 0.03, MB,max = -8.45 ±0.08 and distance d = 3.2±0.3 kpc, using the interstellar extinction E(B−V ) = 0.184±0.035. The distance to the nova found by different methods is in the range 2.7 4.54 kpc. The white dwarf mass was estimated from MB,max as Mwd = 1.04±0.02 M⊙. We suggest that the eruption occurred on the surface of a CO white dwarf. V339 Del is the first nova that has been observed to synthesize the element lithium. The dust consisted of amorphous carbon grains was detected in infrared region. The UBV RCIC light curves of nova constructed from daily means of all available data, including our own observations obtained from its discovery till August 2015, are presented and used to show the track of the nova in the color-color diagram during the first 100 days. The dust particles formed in the dense clumps of ejecta were responsible for the U light curve variability. Our medium-resolution spectrum of V339 Del taken in August 2015 by the 6m SAO telescope in North Caucasus confirms a non-spherical structure of the ejected shell.

We present the results of spectroscopic and photometric studies of a new polar CRTS CSS130604 J 215427+155714, conducted at the telescopes of the SAO RAS. Analysis of the photometric series of observations allowed to clarify the orbital period of the system, P o = 0. d 0672879 (±0.0000003). We build radial velocity curves and trace the intensity variations in the Hβ and Hγ hydrogen lines and He II λ 4686 ˚A ionized heliumline. Based on the Hβ and He II lines we build Doppler maps. It is shown that the line formation region is localized near the Lagrange point. The following parameter estimates of the system are obtained:M 1 = 0.83 ± 0.10M ⊙, M 2 = 0.15 ± 0.01M ⊙, q = M 2/M 1 = 0.18 ± 0.03, i = 53◦ ± 5◦. Based on the results of spectral, photometric and previously published polarimetric observations the possible geometric model of the system is discussed.

The thermodynamic characteristics of hydrogen gas during the transition from optically thin at line frequencies to a state of thermalization without external radiation sources are studied. The radiative terms in the stationary equations describing discrete transitions between atomic levels are included in the Sobolev approximation. It is shown that the transition to a state of thermalization with an increase in the optical thickness of the gas at the frequencies of spectral lines can be accompanied by a strong (by almost a factor of 100) increase in the degree of ionization of the gas. Electron-impact ionization from excited levels predominates in this process.

Results are reported from multicolor photometric observations of the SU UMa-type dwarf nova 1RXS J003828.7+250920 in 2011, roughly a year after the super-outburst of 2010, and in 2012, when the object was in a quiescent state and underwent an outburst. Partial eclipses were observed in the system with depths of about 0m.6 in all color bands, both in the quiescent state and during the outburst. The orbital period of the system, 0.09451001(4) days, was determined for the first time and identifies the object as a dwarf nova in the “gap” in the orbital-period distribution of the cataclysmic variables. The ephemerides for the middle of the eclipse are determined. The average light curve profile resembles the “classical” light curve of U Gem. The orbital periodicity in the brightness variations of 1RXS J003828.7+250920 coexists with another signal with a period of 0.092 days, which we interpret as a period of negative superhumps. The color temperatures derived from the color indices may indicate multicomponent emission from 1RXS J003828.7+250920 by sources which appear to be hot and cold parts of the accretion disk and a hot spot on the accretion disk.

V616 Mon = A0620-00 is a prototype of black hole transient X-ray binaries. Our 2003-16 optical photometry of the object during X-ray quiescence, obtained by 50-250 cm telescopes in Crimea, Caucasus Mountains and Slovakia, consists of ~ 7660 CCD frames in Johnson-Cousins $V,R,R_C,I$ bands and the integral light. During 2003, 2008-9 and 2015-16 passive states, the phase light curve of the binary exhibited mainly variations caused by an ellipsoidal shape of the red dwarf component. During 2004-6 and 2009-14 active states a significant aperiodic broad-band variability (flickering) was present, arising in a black hole accretion disk and a bright spot, where the mass transfer stream hits the outer edge of the disk. Long term photometry of our minima times, together with available positions of superior conjunctions of the red dwarf found from spectroscopy, allowed us to refine the orbital period of V616 Mon to 0.32301407(5) days.

We present the results of the study of the eclipsing polar CRTS CSS081231 J071126+440405. Photometric observations allowed us to refine the orbital period of the system \(P_ \circ = 0_ \cdot ^d 0.08137673\). Considerable changes in the appearance of the object’s spectra have occurred over the period of September 20–21, 2001: the slope of the continuum changed from “red” to “blue”, and the variability of the line profiles over the duration of the orbital period has also changed. Doppler maps have shown a shift of the emission line-forming region along the accretion stream closer to the white dwarf. We measured the duration of the eclipse of the system and imposed constraints on the inclination angle \(78_ \cdot ^ \circ 7 < i < 79_ \cdot ^ \circ 3\). The derived radial velocity amplitude was used to obtain the basic parameters of the system: M 1 = 0.86 ± 0.08M ⊙, M 2 = 0.18 ± 0.02 M ⊙, q = 0.21 ± 0.01, R L2 = 0.20 ± 0.03 R ⊙, A = 0.80 ± 0.03 R ⊙. The spectra of the object exhibit cyclotron harmonics. Their comparison with model spectra allowed us to determine the parameters of the accretion column: B = 31–34 MG, T e = 10–12 keV, θ = 80–90°, and Λ = 105.