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Pilarcik L.,Charles University | Wolf M.,Charles University | Dubovsky P.A.,Vihorlat Observatory | Hornoch K.,Czech Republic Astronomical Institute | Kotkova L.,Czech Republic Astronomical Institute
Astronomy and Astrophysics

The cataclysmic variable star EX Dra is a relatively faint but frequently investigated eclipsing dwarf nova. In total 35 new eclipses were measured photometrically as part of our long-term monitoring of interesting eclipsing systems. Using published and new mid-eclipse times obtained between 2004 and 2011 we constructed the observed-minus-calculated diagram. The current data present 21 years of period modulation with a semi-amplitude of 2.5 min. The eclipse timings show significant deviations from the best sinusoidal fit, which indicates that this ephemeris is not a complete description of the data. The fractional period change is roughly ΔP/P = 3 × 10 -6. © 2012 ESO. Source

Beuermann K.,University of Gottingen | Breitenstein P.,Westfalen Kolleg | Bski B.De.,Jagiellonian University | Diese J.,Max Planck Gymnasium | And 19 more authors.
Astronomy and Astrophysics

We report new mid-eclipse times of the two close binaries NSVS14256825 and HS0705+6700, harboring an sdB primary and a low-mass main-sequence secondary. Both objects display clear variations in their measured orbital period, which can be explained by the action of a third object orbiting the binary. If this interpretation is correct, the third object in NSVS14256825 is a giant planet with a mass of roughly 12 M Jup. For HS0705+6700, we provide evidence that strengthens the case for the suggested periodic nature of the eclipse time variation and reduces the uncertainties in the parameters of the brown dwarf implied by that model. The derived period is 8.4 yr and the mass is 31 M Jup, if the orbit is coplanar with the binary. This research is part of the PlanetFinders project, an ongoing collaboration between professional astronomers and student groups at high schools. © 2012 ESO. Source

Kreibikova Z.,Slovak Academy of Sciences | Parimucha S.,University of P.J. Safarik | Dubovsky P.,Vihorlat Observatory
Contributions of the Astronomical Observatory Skalnate Pleso

Our BV (RI)C photometric observations of Nova Cas 1995 (V723 Cas) more than 15 years after its outburst showed that the photometric vari- ations with the period of 0.693289 days presented in all passbands could be interpreted as pulsations of RR Lyr type and not as an orbital period proposed by several authors earlier. It is most probable that the pulsations occur in the pseudo-atmosphere around the white dwarf which is reminiscent of an F-type star. Source

Kimura M.,Kyoto University | Kato T.,Kyoto University | Imada A.,Kyoto University | Ikuta K.,Kyoto University | And 7 more authors.
Publications of the Astronomical Society of Japan

In 2015 March, the notable WZ Sge-type dwarf nova AL Com exhibited an unusual outburst with a recurrence time of ∼ 1.5 yr, which is the shortest interval of superoutbursts among WZ Sge-type dwarf novae. Early superhumps in the superoutburst light curve were absent, and a precursor was observed at the onset of the superoutburst for the first time in WZ Sge-type dwarf novae. The present superoutburst can be interpreted as a result of the condition that the disk radius barely reached the 3:1 resonance radius, but did not reach the 2:1 resonance one. Ordinary superhumps immediately grew following the precursor. The initial part of the outburst is indistinguishable from those of superoutbursts of ordinary SU UMa-type dwarf novae. This observation supports the interpretation that the 2:1 resonance suppresses a growth of ordinary superhumps. The estimated superhump period and superhump period derivative are Psh = 0.0573185(11) d and Pdot = +1.5(3.1) × 10-5, respectively. These values indicate that the evolution of ordinary superhumps is the same as that in past superoutbursts with much larger extent. Although the light curve during the plateau stage was typical for an SU UMa-type dwarf nova, this superoutburst showed a rebrightening, together with a regrowth of the superhumps. The overall light curve of the rebrightening was the almost the same as those observed in previous rebrightenings. This implies that the rebrightening type is inherent in the system. © 2016 The Author 2016. Published by Oxford University Press on behalf of the Astronomical Society of Japan. Source

Hric L.,Slovak Academy of Sciences | Kundra E.,Slovak Academy of Sciences | Dubovsky P.,Vihorlat Observatory
Contributions of the Astronomical Observatory Skalnate Pleso

V471 Tau is a semi-detached binary classified as a pre-cataclysmic system of a post-common envelope consisting of a white dwarf and a main sequence star of the spectral type K2 V. The eclipsing system is characterized by very fast decreases to the minima as well as the steep increases from minima. The change of brightness caused by the eclipse of the white dwarf lasts only 55 seconds and the total eclipse from the whole orbital period (0.5 day) takes 49 minutes, which is 0.07 of the phase. This behaviour of the system complicates obtaining of eclipses of the high quality from which we can determine the time of minima with expected precision. In the past we tried to observe similar fast changes of brightness by high speed photometry at the expense of a signal-to-noise ratio. In this work we present minima of V471 Tau obtained by CCD photometry that enabled to obtain seven time minima with quite high precision. Whereas the (O-C) diagram of V471 Tau shows long term and gentle changes of values (on average 19 seconds per year), it was therefore necessary to use as the basic time system the heliocentric ephemeris Julian date. We used the time of minima corrected by the difference between the two time systems to construct the (O-C) diagram. Whereas during our observations the (O-C) diagram trend changed we could determine the right model of V471 Tau system, interpret the changes of the (O-C) diagram by a third body in the system and determine the geometry of its orbit as well as the mass function. Source

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