We present an observational study of superoutbursts in six AM CVn systems with orbital periods longer than 35 min. We focused on this range because such systems are expected to have very low mass ratios, making them particularly likely to exhibit double superoutbursts and fading-tail profiles. Using time-resolved photometry from the Variable Star Network and the American Association of Variable Star Observers campaigns, complemented by survey data from the Zwicky Transient Facility, the All-Sky Automated Survey for Supernovae, the Asteroid Terrestrial-impact Last Alert System, Gaia, and the Transiting Exoplanet Survey Satellite, we analyzed nine superoutbursts observed between 2014 and 2021. Among them, four events exhibited clear double superoutburst profiles, analogous to those seen in hydrogen-rich WZ Sge-type dwarf novae. We characterized the fading tail following the second superoutburst and found that it can be divided into three distinct phases (tails A, B, and C) with different power-law fading indices. The effective superoutburst durations were typically 7-13 d, while the dip duration is significantly different among systems. Notably, the effective durations of the second superoutbursts in the double superoutburst were consistently around 5 d. These results highlight the importance of thermal-tidal instabilities in explaining the outbursts of long-orbital-period AM CVn stars. They also suggested that the three-phase fading-tail structure may be a universal feature of the systems and further imply that in systems with short supercycles the effective viscosity during quiescence could be higher than expected from a true quiescent state.

The spectral and photometric studies of the cataclysmic variable Gaia 19cwm (or ZTF19aamkwxk) have been performed. Based on the analysis of long-term variability, it is concluded that the object belongs to WZ Sge type stars. The light curves show eclipses recurring with an orbital period of 86.32048± 0.00005 min, as well as an out-of-eclipse variability with a period of ≈6.45 min. The latter period is stable for ∼4 years and appears to correspond to the rotation of a magnetic white dwarf, i.e., Gaia 19cwm is an intermediate polar. The Gaia 19cwm spectra show photospheric lines of the white dwarf, and Doppler tomograms demonstrate the presence of an accretion disk and a hot spot. Analysis of the eclipse light curve gives an estimates of the white dwarf mass M_1=0.66± 0.06 M_⊙ , the donor mass M_2=0.073± 0.015 M_⊙ , and the orbital inclination i=83.8^∘± 1.1^∘ . Modeling of the spectral energy distribution gives the white dwarf temperature of T_eff≈ 13 000 K. The X-ray luminosity L_X=(1.6± 0.3)× 10^31 erg/s allows to assign Gaia 19cwm to a small group of low-luminosity intermediate polars.

We present a result of the multi-longitude campaign on a photometric investigation of the SU UMa-type dwarf nova in the period gap, V1006 Cyg in 2023.It displayed a long-lasted standstill (a feature of Z Cam-type stars) terminated by a long outburst (a feature of IW And stars).The long outburst did not have superhumps (a feature of SS Cyg-type stars) but showed orbital 0.09837(22) d periodicity instead.Color-color diagrams indicate that layers of the disk of different temperatures contribute to the total radiation of the system.In particular, a large contribution from the innermost hot layers is detected in quiescence.

This paper is an analysis of data from multicolor photometric observations of the asynchronous polar IGR J19552+0044 obtained over 59 nights from 2019-2022 at the Crimean Astrophysical Observatory, as well as at the Terskol Peak, Sanglokh, and Lesniki Observatories, and data from the TESS space telescope obtained on 27 days in 2022. It is shown that the maximum amplitude of the brightness oscillations at the rotation period of the white dwarf ( 2m.5-3m) is observed in the Ic band and is essentially close to zero in the B band. The rotation period of the white dwarf is refined to 0.05645350(14) days. A dependence of the amplitude of the brightness curve of the rotation period on the phase of the synodic period is detected.

В статье приведен анализ данных многоцветных фотометрических наблюдений асинхронного поляра IGR J19552+0044, полученных в течение 59 ночей c 2019 по 2022гг. в Крымской астрофизической обсерватории, а также в обсерваториях Пик Терскол, Санглох, Лесники и данных космического телескопа TESS, полученных в течение 27 сут в 2022г. Показано, что максимальная амплитуда колебаний блеска с периодом вращения белого карлика (~2m.5-3m) наблюдается в полосе Ic и практически близка к нулю в полосе B. Уточнен период вращения белого карлика, составляющий 0.05645350(14) сут. Обнаружена зависимость амплитуды кривой блеска периода вращения от фазы синодического периода.

We report the TESS observations of the SU UMa-type dwarf nova V503 Cyg in 2019 and 2021 during a superoutburst, normal outbursts and quiescence. We identified stage A of the growing superhumps and stage B in positive superhump evolution. We found that the stage A lasted similar to 0.8 d; during stage B a mean period of 0.081367(93) d decreased with the derivative P-dot/P = -4.8* 10(-5). We estimated the mass ratio q = 0.195 using a fractional period excess for stage B. The quiescent state was represented by a strong signal at negative superhumps and a weak one at the orbital period. We detected a gradual increase of the 0.076-d period of negative superhumps in quiescence preceding normal outbursts in 2019 and 2021, and an abrupt decrease of the period during normal outbursts. This means that the radius of the accretion disk increases sharply during the outburst and gradually decreases towards the onset of the next outburst. Such behavior is consistent with the thermal-tidal instability model. We found that the light curve folded on the orbital period shows a profile that changes from double-peak to single-peak, respectively, in the tilted and non-tilted state of the accretion disk. We speculate that this difference may be caused by a state of the accretion disk: in the tilted disk state, one spot is the hot spot on the edge of the disk, while another one may be a spot caused by the matter hitting the inner disk. During the non-tilted disk state, there is one spot on its edge.

We present the results of our optical study of the eclipsing polar Gaia23cer. We analyze the orbital brightness variability in high ( ⟨ r⟩≈ 16.5^m ) and low ( ⟨ r⟩≈ 19.2^m ) states. The system has an orbital period P_orb=102.0665± 0.0015 min and exhibits deep eclipses with a duration Δ t_ecl=401.30± 0.81 s. The spectra have a red cyclotron continuum with the Zeeman H α absorption triplet forming in a magnetic field with a strength B=15.2± 1.1 MG. The source of emission lines has a high radial velocity semi-amplitude ( K≈ 450 km s ^-1 ), and its eclipse lags behind the eclipse of the white dwarf. The mass M_1=0.79± 0.03 M_⊙ and temperature T=11 350± 650 K of the white dwarf have been estimated by modelling the spectral energy distribution. The eclipse duration corresponds to a donor mass M_2=0.10-0.13 M_⊙ and an orbital inclination i=84.3^∘-87.0^∘ . The donor temperature was estimated to be T≈ 2900 K by modelling the elliptical variability and eclipse depth.

The multicolour photometric observations of the dwarf nova IK Leo in 2024, carried out at the Crimean Astrophysical Observatory, are presented. We obtained a corrected value of the superhump period during the outburst, which is 0.055953 days, and studied the evolution of its changes from the plateau stage to the slow decay of the outburst. Changes in the behaviour of colour indices during the outburst are considered.

Superoutbursts in WZ Sge-type dwarf novae (DNe) are characterized by both early superhumps and ordinary superhumps originating from the 2:1 and 3:1 resonances, respectively. However, some WZ Sge-type DNe show a superoutburst lacking early superhumps; it is not well established how these differ from superoutbursts with an early superhump phase. We report time-resolved photometric observations of the WZ Sge-type DN V627 Peg during its 2021 superoutburst. The detection of ordinary superhumps before the superoutburst peak highlights that this 2021 superoutburst of V627 Peg, like that {in} 2014, did not feature an early superhump phase. The duration of stage B superhumps was slightly longer in the 2010 superoutburst accompanying early superhumps than that in the 2014 and 2021 superoutbursts which lacked early superhumps. This result suggests that an accretion disk experiencing the 2:1 resonance may have a larger mass at the inner part of the disk and hence take more time for the inner disk to become eccentric. The presence of a precursor outburst in the 2021 superoutburst suggests that the maximum disk radius should be smaller than that of the 2014 superoutburst, even though the duration of quiescence was longer than that before the 2021 superoutburst. This could be accomplished if the 2021 superoutburst was triggered as an inside-out outburst or if the mass transfer rate in quiescence changes by a factor of two, suggesting that the outburst mechanism and quiescence state of WZ Sge-type DNe may have more variety than ever thought.

We describe, for the first time, the rattling call of the Crested Honey Buzzard Pernis ptilorhynchus. This was previously considered to be a unique vocalization of European Honey Buzzard Pernis apivorus; however, we detected this series of rapidly repeated notes given by an adult female, a second-year, a juvenile and a nestling Crested Honey Buzzard. The rattling calls appeared to be given at times of great excitement, such as to an approaching intruder, mate or parent at the nest, or to a familiar person who usually gave food in the case of captive individuals. The rattling call of an adult female Crested Honey Buzzard at the nest was at a frequency of 2.12 (s.d.+/- 0.07) kHz, and lasted 141 (s.d.+/- 18) ms, which was higher and longer than in European Honey Buzzard.

Photometric studies of the dwarf nova ASASSN-19fy in the “period gap” were carried out in 2020-2021 over 24 nights at the Crimean Astrophysical Observatory of the Russian Academy of Sciences and 3 nights at the Sanglokh International Astronomical Observatory of the Institute of Astrophysics, National Academy of Sciences of Tajikistan. The observations covered a superoutburst, two successive rebrightenings, and a slow return to the pre-outburst (quiescent) state. During this time, superhumps were observed, in the evolution of which the stages of developed superhumps “B,” their damping “C,” and a transition between them were identified. The average period of the superhumps in stage “B” was 0.09278(13) days and it was found its increase during this stage at a rate of (dP / dT) / P = 10·10 –5 . In stage “C” the period of the superhumps was equal to 0.092289(15) days. It is shown that ASASSN-19fy is the twelfth object to join the group of long-period dwarf novae similar to WZ Sge-type stars.

Фотометрические исследования карликовой новой в "провале" периодов ASASSN-19fy были проведены в 2020-2021гг. в течение 24 ночей в Крымской астрофизической обсерватории РАН и трех ночей в Международной астрономической обсерватории Санглох Института астрофизики НАН Республики Таджикистан. Наблюдения охватили сверхвспышку, два повторных поярчания и медленное возвращение к довспышечному состоянию. В течение этого времени наблюдались сверхгорбы, в эволюции которых были выделены стадии развитых сверхгорбов "В", их затухания "С" и переход между ними. Photometric investigations of the dwarf nova ASASSN-19fy in the period gap were carried out in 2020-2021 during 24 nights in the Crimean astrophysical observatory of RAS and 3 nights in Sanglokh International astronomical observatory of Institute of Astrophysics, National Academy of Sciences of Tajikistan. Observations covered superoutburst, two rebrightenings and slow return to quiescence. During this time superhumps have been observed. The stage of developed superhumps "B", their dying stage "C" as well as transition between them were identified.

We made a time-resolved photometric campaign of the bright cataclysmic variable ASAS J071404+7004.3 in 2020. Inight et al. (2022, arXiv/2109.14514) recently published time-resolved optical spectroscopy, X-ray observations and long- and short-term optical variations. Although their results were correct in many parts, they classified ASAS J071404+7004.3 as a VY Scl-type novalike object. By comparing the ASAS-SN data of this object and the IW And-type object HO Pup, we showed that their type classification was incorrect. ASAS J071404+7004.3 showed outbursts from a standstill followed by shallow dips, which is the defining characteristic of an IW And star. This object predominantly showed states with low-amplitude dwarf nova-type oscillations, some of which could be identified as the "heartbeat"-type state as a variety of the IW And-type phenomenon. The low state described by Inight et al. (2022) was not a true low state of a VY Scl star, but a dwarf nova-type state with increased outburst amplitudes. Both ground-based (our campaign) and TESS observations detected orbital variations whose periods [0.136589(5) d by the ground-based campaign and 0.1366476(3) d by the TESS data] are in very good agreement with the one obtained by radial-velocity studies by Inight et al. (2022). The standstill in 2019-2020 in ASAS J071404+7004.3 was not brighter than its dwarf nova-type states. The brightest moment of this object occurred when the amplitudes of dwarf nova-type variations were large, which challenges the widely accepted interpretation that standstills in Z Cam stars occur when the mass-transfer rates are high.

Hill et al. (2022, arXiv:2203.00221) recently published analysis of the intermediate polar DO Dra (YY Dra) using TESS, ASAS-SN and ZTF data. They also attempted a search for the “lost” variable YY Dra using modern catalogs of variable stars and found no corresponding one. This search drew our renewed attention and we studied the original discovery paper of YY Dra by Tsesevich (Zessewitch). We found that two out of four variables reported by him were either lost or improperly studied. The coordinate offset from the correct position in another object was much larger than the expected error. Using the information of the published period and epoch of YY Dra, we suspect that Tsesevich used a couple of plates on which the object was invisible to derive the period rather than from a completely phased light curve. DO Dra sometimes spends high states around 14 mag for a month to several months and it would not be surprising if Tsesevich observed DO Dra in such a state and suspected it to be an Algol, urging him to examine the plate archive to obtain the moments when the variable was undetected. We suspect that Tsesevich indeed observed DO Dra, rather than a different, lost eclipsing variable. The final conclusion of this matter is almost impossible to reach due to the consequence of the World War II, destructing the large part of plate collections. Until the humanity becomes wise enough to restore this destruction and wise enough to be able to avoid further destruction of human achievements, the designation YY Dra would better be conserved as a record of our negative history and unambiguous DO Dra would better be used to designate the intermediate polar 3A 1148+719.

The analysis of the white dwarf pulsations of the dwarf nova EZ Lyn (which outbursted in 2010) in 2015–2020 years basedon photometry at 2.6-m Shajn telescope of the Crimean Astrophysical Observatory is presented. It revealed an increasein the period of pulsations from 234 to 756 sec. In accordance with the empirical relationship between the period of thepulsations and the temperature of the white dwarf, this means a fast cooling of the white dwarf after the 2010 outburst.

SS Cyg has long been recognized as the prototype of a group of dwarf novae that show only outbursts. However, this object has entered a quite anomalous event in 2021, which at first appeared to be standstill, i.e., an almost constant luminosity state, observed in Z Cam-type dwarf novae. This unexpected event gives us a great opportunity to reconsider the nature of standstill in cataclysmic variables. We have observed this anomalous event and its forerunner, a gradual and simultaneous increase in the optical and X-ray flux during quiescence, through many optical telescopes and the X-ray telescopes NICER and NuSTAR. We have not found any amplification of the orbital hump during quiescence before the anomalous event, which suggests that the mass transfer rate did not significantly fluctuate on average. The estimated X-ray flux was not enough to explain the increment of the optical flux during quiescence via X-ray irradiation of the disk and the secondary star. It would be natural to consider that viscosity in the quiescent disk was enhanced before the anomalous event, which increased mass accretion rates in the disk and raised not only the optical flux but also the X-ray flux. We suggest that enhanced viscosity also triggered the standstill-like phenomenon in SS Cyg, which is considered to be a series of small outbursts. The inner part of the disk would always stay in the outburst state and only its outer part would be unstable against the thermal-viscous instability during this phenomenon, which is consistent with the observed optical color variations. This scenario is in line with our X-ray spectral analyses which imply that the X-ray emitting inner accretion flow became hotter than usual and vertically expanded and that it became denser and was cooled down after the onset of the standstill-like state.

IW And stars are a recently recognized subgroup of dwarf novae which are characterized by (often repetitive) slowly rising standstills terminated by brightening, but the exact mechanism for this variation is not yet identified. We have identified BO Cet, which had been considered as a novalike cataclysmic variable, as a new member of IW And stars based on its behavior in 2019-2020. In addition to this, the object showed dwarf nova-type outbursts in 2020-2021, and superhumps that had periods 7.8% longer than the orbital one developed during at least one long outburst. This object has been confirmed as an SU UMa-type dwarf nova with an exceptionally long orbital period (0.1398d). BO Cet is thus the first cataclysmic variable showing both SU UMa-type and IW And-type features. We obtained a mass ratio (q) of 0.31-0.34 from the superhumps in the growing phase (stage A superhumps). At this q, the radius of the 3:1 resonance, responsible for tidal instability and superhumps, and the tidal truncation radius are very similar. We interpret that in some occasions this object showed IW And-type variation when the disk size was not large enough, but that the radius of the 3 : 1 resonance could be reached as a result of thermal instability. We also discuss that there are SU UMa-type dwarf novae above q = 0.30, which is above the previously considered limit (similar to 0.25) derived from numerical simulations and that this is possible since the radius of the 3: 1 resonance is inside the tidal truncation radius. We constrained the mass of the white dwarf larger than 1.0 M-circle dot, which may be responsible for the IW And-type behavior and the observed strength of the He II emission. The exact reason, however, why this object is unique in that it shows both SU UMa-type and IW And-type features is still unsolved.

We report on the multi-wavelength photometry of the 2018 superoutburst in EG Cnc. We have detected stage A superhumps and long-lasting late-stage superhumps via the optical photometry and have constrained the binary mass ratio and its possible range. The median value of the mass ratio is 0.048 and the upper limit is 0.057, which still implies that EG Cnc is one of the possible candidates for period bouncers. This object also showed multiple rebrightenings in this superoutburst which are the same as those in its previous superoutburst in 1996–1997, despite the difference in the main superoutburst. This would represent that the rebrightening type is inherent to each object and is independent of the initial disk mass at the beginning of superoutbursts. We also found that B − I and J − Ks colors were unusually red just before the rebrightening phase and became bluer during the quiescence between rebrightenings, which would mean that the low-temperature mass reservoir at the outermost disk accreted with time after the main superoutburst. Also, the ultraviolet flux was sensitive to rebrightenings as well as the optical flux, and the U − B color became redder during the rebrightening phase, which would indicate that the inner disk became cooler when this object repeated rebrightenings. Our results thus basically support the idea that the cool mass reservoir in the outermost disk is responsible for rebrightenings.

ASASSN-V J205543.90+240033.5 has been suggested to be a white dwarf pulsar by Kato (2021, arXiv:2108.09060). We obtained time-resolved photometry and identified the orbital and spin periods to be 0.523490(1) d and 0.00678591(1) d = 9.77 min, respectively. These values strengthen the similarity of this object with AR Sco. We estimated that the strength of the spin pulse is 3.6 times smaller than in AR Sco.