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Lampens P.,Koninklijke Sterrenwacht Van Belgie | Strigachev A.,Bulgarian Academy of Science | Kim S.-L.,Korea Astronomy and Space Science Institute KASI | Rodriguez E.,Institute Astrofisica Of Andalucia | And 15 more authors.
Astronomy and Astrophysics | Year: 2011

We present the results of a multi-site photometric campaign carried out in 2004-2008 for the Algol-type eclipsing binary system CT Her, the primary component of which displays δ Scuti-type oscillations. Our data consist of differential light curves collected in the filters B and V, which were analysed using the method of Wilson-Devinney (Phoebe). After identifying an adequate binary model and removing the best-fit light-curve solution, we performed a Fourier analysis of the residual B and V light curves to investigate the pulsational behaviour. We confirm the presence of rapid pulsations with a main period of 27.2 min. Up to eight significant frequencies with semi-amplitudes in the range 3 to 1 mmag were detected, all of which lie in the frequency range 43.5-53.5 d-1. This result is independent of the choice of the primary's effective temperature (8200 or 8700 K) since the light-curve models of the binary are very similar in both cases. This is yet another case of a complex frequency spectrum observed for an accreting δ Scuti-type star (after Y Cam). In addition, we demonstrate that the amplitudes of several pulsation frequencies provide evidence of variability on timescales as short as 1-2 years, perhaps even less. Moreover, our analysis takes into account some recently acquired spectra, from which we obtained the corresponding radial velocities for the years 2007-2009. Investigation of the O-C diagram shows that further monitoring of the epochs of eclipse minima of CT Her will cast new light on the evolution of its orbital period. © 2011 ESO.

Ohshima T.,Kyoto University | Kato T.,Kyoto University | Pavlenko E.P.,Crimean Astrophysical Observatory | Itoh H.,Variable Star Observers League in Japan VSOLJ | And 19 more authors.
Publications of the Astronomical Society of Japan | Year: 2012

We report on a discovery of "negative" superhumps during the 2011 January superoutburst of ERUMa. During the superoutburst, which started on 2011 January 16, we detected negative superhumps having a period of 0.062242(9) d, shorter than its orbital period by 2.2%. No evidence of a positive superhump was detected during this observation. This finding indicates that the disk exhibited retrograde precession during this superoutburst, contrary to all other known cases of superoutbursts. The duration of this superoutburst was shorter than those of ordinary superoutbursts, and the interval of its normal outburstswas longer than those of ordinary normal outbursts of ER UMa. We suggest the possibility that such unusual outburst properties are likely to be a result of a disk tilt, which is supposed to be a cause of negative superhumps; the tilted disk could prevent the disk from being filled with materials in the outmost region, which is supposed to be responsible for long-duration superoutbursts in ERUMa-type dwarf novae. This discovery signifies the importance of the classical prograde precession in sustaining long-duration superoutbursts. Furthermore, the presence of pronounced negative superhumps in this system with a high mass-transfer rate supports the hypothesis that hydrodynamical lift is the cause of the disk tilt. © 2012. Astronomical Society of Japan.

Nakata C.,Kyoto University | Kato T.,Crimean Astrophysical Observatory | Nogami D.,Kyoto University | Pavlenko E.P.,University of Huelva | And 14 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

We report on photometric observations of two dwarf novae, OTJ075418.7+381225 and OTJ230425.8+062546, which showed superoutbursts in 2013 (OTJ075418) and in 2011 (OTJ230425). Their mean periods of the superhump were 0.0722403(26)d (OTJ075418) and 0.067317(35)d (OTJ230425). These objects showed a very long growth stage of the superhump (stage A) and a large period decrease in the stage A-B transition. The long stage A suggests slow evolution of the superhump due to the very small mass ratio of these objects. The declining rates during the plateau phase in the superoutburst of these objects were lower than those of SUUMa-type dwarf novae (DNe) with a similar superhump period. These properties were similar to those of SSSJ122221.7-311523, the most likely candidate for the period bouncer. Therefore, these two DNe are regarded as likely candidates for the period bouncer. We estimated the number density of period bouncers roughly from our observations for the last five years. There is a possibility that these WZSge-type DNe with unusual outburst properties might account for the missing population of the period bouncer suggested by the evolutionary scenario. © 2014 © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.

Pavlenko E.P.,Crimean Astrophysical Observatory | Kato T.,Virtual Telescope Project | Antonyuk O.I.,Crimean Astrophysical Observatory | Ohshima T.,Variable Star Observers League in Japan VSOLJ | And 15 more authors.
Publications of the Astronomical Society of Japan | Year: 2013

We present a photometric study of NYSer, an in-the-gap SUUMa-type nova, in 2002 and 2013. We determined the duration of its superoutburst and the mean period of its superhump are 18d and 0.10458d, respectively. We detected in 2013 that NYSer showed two distinct states separated by a superoutburst. A state of rather infrequent normal outbursts lasted at least 44d before the superoutburst, and a state of frequent outbursts started immediately after the superoutburst and lasted at least 34d. Unlike a typical SUUMa star with a bimodal distribution of outburst duration, NYSer displayed a diversity of normal outbursts. In the state of infrequent outbursts, we detected a wide ∼12d outburst accompanied by 0.098d orbital modulation but without superhumps ever established in NYSer. We classified this as a "wide normal outburst." The orbital period was dominant both in quiescence and during normal outbursts in this state. In the state of the most frequent normal outbursts, the 0.10465d positive superhump period was dominant and coexisted with the orbital modulation. In 2002 we detected the normal outburst of "intermediate" 5-6d duration that was also accompanied by orbital modulations. © 2014 © The Author 2014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.

Kato T.,Kyoto University | Hambsch F.-J.,Groupe Europeen dObservations Stellaires GEOS | Dubovsky P.A.,Vihorlat Observatory | Kudzej I.,Vihorlat Observatory | And 89 more authors.
Publications of the Astronomical Society of Japan | Year: 2015

Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae, observed mainly during the 2014-2015 season, and characterized these objects. Our project has greatly improved the statistics of the distribution of orbital periods, which is a good approximation of the distribution of cataclysmic variables at the terminal evolutionary stage, and has confirmed the presence of a period minimum at a period of 0.053 d and a period spike just above this period. The number density monotonically decreased toward the longer period and there was no strong indication of a period gap. We detected possible negative superhumps in Z Cha. It is possible that normal outbursts are also suppressed by the presence of a disk tilt in this system. There was no indication of enhanced orbital humps just preceding the superoutburst, and this result favors the thermal-tidal disk instability as the origin of superoutbursts. We detected superhumps in three AM CVn-type dwarf novae. Our observations and recent other detections suggest that 8% of objects showing dwarf nova-type outbursts are AM CVn-type objects. AM CVn-type objects and EI Psc-type objects may be more abundant than previously recognized. OT J213806, a WZ Sge-type object, exhibited remarkably different features between the 2010 and 2014 superoutbursts. Although the 2014 superoutburst was much fainter, the plateau phase was shorter than the 2010 one, and the course of the rebrightening phase was similar. This object indicates that the O - C diagrams of superhumps can indeed be variable, at least in WZ Sge-type objects. Four deeply eclipsing SU UMa-type dwarf novae (ASASSN-13cx, ASASSN-14ag, ASASSN-15bu, and NSV 4618) were identified. We studied long-term trends in supercycles in MM Hya and CY UMa and found systematic variations of supercycles of ∼20%. © The Author 2015. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.

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