In this work, we investigate the eclipse timing of the polar binary HU Aquarii that has been observed for almost two decades. Recently, Qian et al. attributed large (O-C) deviations between the eclipse ephemeris and observations to a compact system of two massive jovian companions. We improve the Keplerian, kinematic model of the Light Travel Time (LTT) effect and re-analyse the whole currently available data set. We add almost 60 new, yet unpublished, mostly precision light curves obtained using the time high-resolution photo-polarimeter OPTIMA, as well as photometric observations performed at the MONET/N, PIRATE and TCS telescopes. We determine new mid--egress times with a mean uncertainty at the level of 1 second or better. We claim that because the observations that currently exist in the literature are non-homogeneous with respect to spectral windows (ultraviolet, X-ray, visual, polarimetric mode) and the reported mid--egress measurements errors, they may introduce systematics that affect orbital fits. Indeed, we find that the published data, when taken literally, cannot be explained by any unique solution. Many qualitatively different and best-fit 2-planet configurations, including self-consistent, Newtonian N-body solutions may be able to explain the data. However, using high resolution, precision OPTIMA light curves, we find that the (O-C) deviations are best explained by the presence of a single circumbinary companion orbiting at a distance of ~4.5 AU with a small eccentricity and having ~7 Jupiter-masses. This object could be the next circumbinary planet detected from the ground, similar to the announced companions around close binaries HW Vir, NN Ser, UZ For, DP Leo or SZ Her, and planets of this type around Kepler-16, Kepler-34 and Kepler-35.Comment: 20 pages, 18 figures, accepted to Monthly Notices of the Royal Astronomical Society (MNRAS

We study the mid-egress eclipse timing data gathered for the cataclysmic binary HU Aquarii during the years 1993-2014. The (O-C) residuals were previously attributed to a single ∼ 7 Jupiter mass companion in ∼ 5 au orbit or to a stable 2-planet system with an unconstrained outermost orbit. We present 22 new observations gathered between June, 2011 and July, 2014 with four instruments around the world. They reveal a systematic deviation of ∼ 60-120 seconds from the older ephemeris. We re-analyse the whole set of the timing data available. Our results provide an erratum to the previous HU Aqr planetary models, indicating that the hypothesis for a third and fourth body in this system is uncertain. The dynamical stability criterion and a particular geometry of orbits rule out coplanar 2-planet configurations. A putative HU Aqr planetary system may be more complex, e.g., highly non-coplanar. Indeed, we found examples of 3-planet configurations with the middle planet in a retrograde orbit, which are stable for at least 1 Gyr, and consistent with the observations. The (O-C) may be also driven by oscillations of the gravitational quadrupole moment of the secondary, as predicted by the Lanza et al. modification of the Applegate mechanism. Further systematic, long-term monitoring of HU Aqr is required to interpret the (O-C) residuals.

The cyclic behaviour of (O-C) residuals of eclipse timings in the sdB+M eclipsing binary NSVS 14256825 was previously attributed to one or two Jovian-type circumbinary planets. We report 83 new eclipse timings that not only fill in the gaps in those already published but also extend the time span of the (O-C) diagram by three years. Based on the archival and our new data spanning over more than 17 years we re-examined the up to date system (O-C). The data revealed systematic, quasi-sinusoidal variation deviating from an older linear ephemeris by about 100 s. It also exhibits a maximum in the (O-C) near JD 2,456,400 that was previously unknown. We consider two most credible explanations of the (O-C) variability: the light propagation time due to the presence of an invisible companion in a distant circumbinary orbit, and magnetic cycles reshaping one of the binary components, known as the Applegate or Lanza-Rodonó effect. We found that the latter mechanism is unlikely due to the insufficient energy budget of the M-dwarf secondary. In the framework of the third-body hypothesis, we obtained meaningful constraints on the Keplerian parameters of a putative companion and its mass. Our best-fitting model indicates that the observed quasi-periodic (O-C) variability can be explained by the presence of a brown dwarf with the minimal mass of 15 Jupiter masses rather than a planet, orbiting the binary in a moderately elliptical orbit (e 0.175) with the period of ∼ 10 years. Our analysis rules out two planets model proposed earlier.

As a result of various studies, it has been determined that several post-common envelope eclipsing binaries have variations in their orbital periods. These variations are thought to be caused by the existence of additional bodies in the system (hypothetical stars or planets) and/or other physical effects (such as angular momentum loss, magnetic activity) of the binary system. It is also known that the sdB+M eclipsing system NY Vir has shown such variations in the last decade, indicating additional objects and/or other physical effects. In this work, we present 51 new eclipse times for this system, which extend the time span of it is O − C diagram by about three years, obtained between 2015 and 2021 using two different telescopes in Turkey. The data obtained in the last 3 years shows a new trend in the O − C diagram differently from the predictions of the previous studies. Our model is consistent with the new O − C diagram, which is statistically well fitted with the quadratic term and the additional two planets with masses of M3 = 2.74 MJup and M4 = 5.59 MJup. However, the orbital period variation can also be related to magnetic activity. In order to better understand the mechanism causing the changes in the orbital period, new observation data is needed that will show at least one full cycle of the change in the O-C diagram.

We present fast timing photometric observations of the intermediate polar V2069 Cygni (RX J2123.7+4217) using the Optical Timing Analyzer (OPTIMA) at the 1.3-m telescope of Skinakas Observatory. The optical (450-950 nm) light curve of V2069 Cygni was measured with sub-second resolution for the first time during 2009 July and revealed a double-peaked pulsation with a period of 743.38 ± 0.25 s. A similar double-peaked modulation was found in the simultaneous Swift satellite observations. We suggest that this period represents the spin of the white dwarf accretor. Moreover, we present results from a detailed analysis of the XMM-Newton observation, which also shows a double-peaked modulation, however shifted in phase, with a period of 742.35 ± 0.23 s. The X-ray spectra obtained from the XMM-Newton European Photon Imaging Camera (EPIC) instruments were modelled by a plasma emission and a soft blackbody component with a partial covering photoelectric absorption model with a covering fraction of 0.65. An additional Gaussian emission line at 6.385 keV with an equivalent width of 243 eV is required to account for fluorescent emission from neutral iron. The iron fluorescence (∼6.4 keV) and Fe XXVI lines (∼6.95 keV) are clearly resolved in the EPIC spectra. In the P orb -P spin diagram of intermediate polars, V2069 Cyg shows a low spin-to-orbit ratio of ∼0.0276 in comparison with ∼0.1 for other intermediate polars.

We report the results of new transit observations for the three hot Jupiter-like planets HATP-36b, HATP-56b and WASP-52b respectively. Transit timing variations (TTVs) are presented for these systems based on observations that span the period 2016–2020. The data were collected with the 0.6 m telescope at Adiyaman University (ADYU60, Turkey) and the 1.0 m telescope at TÜBİTAK National Observatory (TUG, Turkey). Global fits were performed to the combined light curves for each system along with the corresponding radial velocity (RV) data taken from the literature. The extracted parameters (for all three systems) are found to be consistent with the values from previous studies. Through fits to the combined mid-transit times data from our observations and the data available in the literature, an updated linear ephemeris is obtained for each system. Although a number of potential outliers are noted in the respective O-C diagrams, the majority of the data are consistent within the 3σ confidence level implying a lack of convincing evidence for the existence of additional objects in the systems studied.

We present fast timing photometric observations of the intermediate polar V2069 Cygni (RX J2123.7+4217) using the Optical Timing Analyzer (OPTIMA) at the Skinakas Observatory 1.3 m telescope. OPTIMA is a single-photon counting aperture photo-polarimeter with the timing accuracy of about 4 microseconds and absolute (GPS) tagging of photon arrival-times. The optical (450-950 nm) light curve of V2069 Cygni was measured with sub-second resolution during July 2009 and revealed a double-peaked pulsation with 743.385 (±0.250) s period. A similar doublepeaked modulation was found in simultaneous soft X-ray observations with the Swift satellite. We suggest that the 743.385 (±0.250) s period represents the spin of the white dwarf accretor. In the P orb -P spin diagram of all IPs, V2069 Cyg is rather an indistinct member of this population. It has however a rather low spin to orbit ratio of ∼ 0.027.

is an eclipsing CV system hosting a highly magnetic white dwarf orbited by a M4V dwarf with a period of about 125 min. Its orbital ephemeris has been followed with precision since 1993. Regular OPTIMA observations of eclipses of HU Aqr since 1999 can be used to follow the secular changes of the binary orbit. We report new modeling of the orbital ephemeris including recent 2008/2009 observations and find that a model including a linear and quadratic term as well as two sinusoidal oscillatory terms provides the best fit to the observed eclipses. We propose that the sinusoidal variations can be explained by the presence of two giant planets in orbit around HU Aqr.