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In three previous papers I showed that the series of the midpoints of the times of all the X-ray flares of Sgr A* that have been detected so far harbor a statistical trend termed pacemaker regularity. This means that X-ray flares are detected more frequently around time points that are a subset of a periodic grid on the time axis of period P_X=0.1032 day=149 minutes. The series of the times of detection of the peaks of near IR (NIR) flares of the object are also regulated by a pacemaker with a period of P_IR=0.028 day=41 minutes. Here, I show that the series of the midpoints of the times of recorded NIR flares are also regulated by a pacemaker of the period P_IRM=0.039 day=56 minutes. The two pacemakers found in the previous papers were interpreted as signals of a star that revolves around the black hole of Sgr A* in orbit with a mean radius of ~3.2 Schwarzschild radii of the black hole, here corrected to ~3.13. The finding of the period of the third pacemaker is consistent with the suggested revolving star model. Here, I present the specific orbit of the star as well as a plausible description of its sidereal rotation. The model also implies that the star has an unusual internal structure. I show that the discovery of the GRAVITY collaboration of the motion of hotspots at distances from the black hole that are of the order of very few Schwarzschild radii of it may well be understood within the context of the revolving star model., ApJ, printed version

The frequent flaring events in the X-ray and the NIR radiation of Sgr A* seem not to be periodic in time. However, statistical regularities, here termed "modulations by a pacemaker", are found in the recorded arrival times of both types of events. The characteristic time of the X-ray pacemaker is 149 min and that of the NIR pacemaker is 40 min. Their reality as derived from observed data can be accepted at larger than 4.6{\sigma} and 3.8{\sigma} levels of statistical confidence, respectively. These results can be interpreted as evidence for a star that revolves around the BH of Sgr A* in a slightly elliptical precessing orbit, at a distance of 3-3.5 Schwarzschild radii of the BH. The period of the X-ray pacemaker, which is not a periodicity of the flare occurrences themselves, is the epicyclic period of the star orbital motion. This is the time interval between 2 successive passages of the star through the peri-center of its orbit. The NIR pacemaker period is the sidereal binary period of the star revolution. The origin of the X-ray flares is in episodes of intense mass loss from the star that occur preferably near the pericenter phase of the binary revolution. The NIR flares originate or are triggered by processes that are internal to the star. The radiation emitted in the direction of Earth is slightly modulated by the changing aspect ratio of the two components of the BH\star binary to the line of sight from Earth at the sidereal binary frequency., Comment: Minor corrections, matching published version

In a recent paper (Leibowitz 2017) I have shown that the 39 large X-ray flares of Sgr A* that were recorded by Chandra observatory in the year 2012, are concentrated preferably around tick marks of an equi-distance grid on the time axis. The period of this grid as found in L1 is 0.1033 days. In this work I show that the effect can be found among all the large X-ray flares recoded by Chandra and XMM-Newton along 15 years. The midpoints of all the 71 large flares recorded between the years 2000 and 2014 are also tightly grouped around tick marks of a grid with this period, or more likely, 0.1032 day. This result is obtained with a confidence level of at least 3.27{\sigma} and very likely of 4.62{\sigma}. I find also a possible hint that a similar grid is underlying IR flares of the object. I suggest that the pacemaker in the occurrences of the large X-ray flares of Sgr A* is a mass of the order of a low mass star or a small planet, in a slightly eccentric Keplerian orbit around the SMBH at the centre of the Galaxy. The radius of this orbit is about 6.6 Schwarzschild radii of the BH., Comment: MNRAS, accepted. Sequel to arXiv:1607.02519

In an intensive observational campaign in the 9 month duration of Chandra X-ray Visionary Project that was conducted in the year 2012, 39 large X-ray flares of Sgr A* were recorded. An analysis of the times of the observed flares reveals that the 39 flares are separated in time by intervals that are grouped around integer numbers times 0.10333 days. This time interval is thus the period of a uniform grid of equally spaced points on the time axis. The grouping of the flares around tic marks of this grid is derived from the data with at least a 3.2 {\sigma} level of statistical significance. No signal of any period can be found among 22 flares recorded by Chandra in the years 2013-2014. If the 0.10333 d period is that of a nearly circular Keplerian orbit around the blackhole at the center of the Galaxy, its radius is at 7.6 Schwarzschild radii. Large flares were more likely to be triggered when the agent responsible for their outbursts was near the peri-center phase of its slightly eccentric orbit., Comment: Accepted for publication in New Astronomy

We analyze an optical light curve of the symbiotic system AG Draconis covering the last 120 years of its history. During the first 32 years the system was in a quiescence state. Around the year 1922 the star's quiescence luminosity brightened by 0.29 mag. The last 82 years of the light curve (LC) are characterized by a series of outbursts of 1-2 magnitude in brightness and about 100 days in duration. The outbursts are distributed along the time axis in 6 clusters with a quasi-periodic cycle of some 5300 days. The time intervals among the outbursts themselves are integral numbers of the period 373.5 days. During quiescence states the LC oscillates with the binary period of the system of 550 d. The LC contains also a weak periodic signal with a period of 350 d, attributed to pulsations of the giant star. Another period of 1160 d is also present in the light curve, being the sidereal rotation period of the giant star. We suggest that the outbursts are events of intense mass transfer from the giant onto the hot component. These are modulated by an interplay between a solar-like magnetic dynamo cycle operating in the outer layers of the giant, and a tidal deformation of these layers that circulates the surface of the giant with the synodic diurnal period of 373.5 Earth days. AG Dra is the 5th symbiotic system with a light curve that reflects such an intense magnetic and magnetically modulated activity. (Abridged)

PG 1351+489 is one of the 20 DBVs ― pulsating helium-atmosphere white dwarf stars ― known and has the simplest power spectrum for this class of star, making it a good candidate to study cooling rates. We report accurate period determinations for the main peak at 489.334 48 s and two other normal modes using data from the Whole Earth Telescope (WET) observations of 1995 and 2009. In 2009, we detected a new pulsation mode and the main pulsation mode exhibited substantial change in its amplitude compared to all previous observations. We were able to estimate the star's rotation period, of 8.9 h, and discuss a possible determination of the rate of period change of (2.0 ± 0.9) x 10 ―13 s s ―1 , the first such estimate for a DBV.

We report on a detailed analysis of the optical light curve of the symbiotic system BX Mon, the data of which were gathered from the literature. The light curve covers the period December 1889 March 2009, with a gap of no observations between March 1940 and February 1972. The light curve is characterized by strong oscillations of peak to peak amplitude of 2 to more than 3 magnitudes. Before the gap the fluctuations were modulated mainly by a period Pa=1373 d, the diurnal cycle of an observer on the surface of the rotating M giant, whose sun is the hot component. After the gap the dominant periodicity is Pb=1256 d, which is the binary period of the system. Higher harmonics as well as a few beats of the two major periodicities can also be identified in the light curve. We identify one of the beat periods, Pr=656 d, as the sidereal rotation period of the giant component of the system. The period switching that took place during the gap in the observations was possibly associated with a certain cataclysmic event, hints of which may be recognized in the LC in the first 11 years after the gap. We suggest that the origin of the major oscillations is in periodic episodes of mass accretion from the M giant onto the hot component of the system. The event of the period switching is possibly related to an intensive magnetic activity in the outer layers of the giant star. [abridged]

We report on the discovery of a coherent periodicity in the B light curve of the symbiotic star BF Cyg. The signal was detected in some sections of the light curve of the star recorded in the year 2003 as double hump periodic variations with an amplitude of ~7 mmag. In the year 2004 the signal was also present in only a subsection of the light curve. In that year, the system was about twice as bright and the amplitude of the oscillations was about half of what it was in 2003. In 2004 the cycle structure was of a single hump, the phase of which coincided with the phase of one of the humps in the 2003 cycle. No periodic signal was detected in a third, short series of observations performed in the year 2007, when the star was three magnitudes brighter than in 2003. We interpret the periodicity as the spin period of the white dwarf component of this interacting binary system. We suggest that the signal in 2003 originated in two hot spots on or near the surface of the white dwarf, most likely around the two antipodes of an oblique dipole magnetic field of this star. Magnetic field lines funneled accreted matter from the wind of the cool component to the pole areas, where the falling material created the hot spots. This process is apparently intermittent in its nature. In 2004, the activity near only one pole was enhanced enough to raise the signal above the threshold of our detection ability., 7 pages, 5 figures, accepted for publication in MNRAS

We have reanalyzed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550 d quasi periodicity characterizes the repetition time of the outburst episodes of this symbiotic star. Six such events have been recorded so far. During quiescence states of the system, i.e. in time intervals between outbursts, the LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the well known orbital period of the Z And binary system that have been measured also spectroscopically. A third coherent period of P3=658.4 d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outbursts phenomenon and the clock that drives it, to a solar type magnetic dynamo cycle that operates in the convection and the outer layers of the giant star of the system. We suggest that the intense surface activity of the giant star during maximum phases of its magnetic cycle is especially enhanced in one or two antipode regions, fixed in the atmosphere of the star and rotating with it. Such spots could be active regions around the North and the South poles of a general magnetic dipole field of the star. The P3 periodicity is half the beat of the binary orbital period of the system and the spin period of the giant. The latter is then either 482 or 1790 d. If only one pole is active on the surface of the giant, P3 is the beat period itself, and the spin period is 352 d. It could also be 5000 d if the giant is rotating in retrograde direction. We briefly compare these findings in the LC of Z And to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.

We present the X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with the XMM-Newton and Chandra observatories. Two weeks after optical maximum, the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2.4 x 10^36 erg s^-1 in the 0.33-10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures. All the lines are blue-shifted, with the velocity shift increasing with lower ionization state and longer wavelength. Two weeks later, the spectrum was still dominated by emission lines, although the line ratios present indicate cooling. During this observation, a soft X-ray flare occurred in which a new system of higher velocity emission lines appeared in the soft end of the spectrum. The dominant component during the third month was the supersoft continuum with the broad absorption features of a hot white dwarf atmosphere. A preliminary model fit indicates a white dwarf temperature slightly exceeding 800,000 K. The X-ray luminosity reached at least 9 x 10^37 erg s^-1 in the 0.2-1 keV range, while the intrinsic nebular absorption decreased by a factor of five since the first observation. The spectral fits indicate a massive white dwarf, with a mass of at least 1.2 M_\odot. Therefore, RS Oph may be an important type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 after the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7 and 8 months after optical maximum dropped by more than two orders of magnitude. The spectra do not appear to consistent with emission from an accretion disk.

We analyze the historical light curve of the symbiotic star YY Her, from 1890 up to December 2005. A secular declining trend is detected, at a rate of ~.01 magn in 1000 d, suggesting that the system could belong to the sub-class of symbiotic novae. Several outburst events are superposed on this slow decline. Three independent periodicities are identified in the light curve. A quasi-periodicity of 4650.7 d is detected for the outburst occurrence. We suggest that it is a signature of a solar-type magnetic dynamo cycle in the giant component. A period of 593.2 d modulates the quiescent light curve and it is identified as the binary period of the system. During outburst events the system shows a stable periodic oscillation of 551.4 d. We suggest that it is the rotation period of the giant.The secondary minima detected at some epochs of quiescence are probably due to dark spots on the surface of the rotating giant. The difference between the frequencies of these two last periods is the frequency of a tidal wave in the outer layers of the giant. A period which is a beat between the magnetic cycle and the tidal wave period is also apparent in the light curve. YY Her is a third symbiotic system exhibiting these cycles in their light curve, suggesting that a magnetic dynamo process is prevalent in the giant components of symbiotic stars, playing an important role in the outburst mechanism of some of these systems., 8 pages, 8 figures. Accepted by MNRAS

The classical nova V4743 Sgr was observed with XMM-Newton for about 10 hours on April 4 2003, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X-ray source ever observed. We present the results of a time series analysis performed on the X-ray light curve obtained in this observation, and in a previous shorter observation done with Chandra 16 days earlier. Intense variability, with amplitude as large as 40% of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 day time interval between the two observations. We suggest that a periods in the power spectrum of both light curves at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf in the system, and that other observed frequencies are signatures of nonradial white dwarf pulsations. A possible signal with a 24000 sec period is also found in the XMM-Newton light curve: a cycle and a half are clearly identified. This period is consistent with the 24278 s periodicity discovered in the optical light curve of the source and thought to be the orbital period of the nova binary system.

International audience; This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 ?Hz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of ?=1 and ?=2 non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately "thick" hydrogen mass fraction log qH ? -7.0. The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.

We present the preliminary results of more than 160 h of time-series photometry on the subdwarf B pulsating star PG 1325+101 (Teff= 34 500 K log g= 6.1), obtained in March-April 2003 at nine different sites. The amplitude spectrum confirms the presence of the four frequencies found in the discovery run at the NOT in July 2001 (but with significantly lower amplitudes in two of them) and also shows at least two new low amplitude signals. Moreover, there is no sign of frequency splitting near the main peak at 7256 μHz, suggesting that this mode might be actually radial as indicated by the time-resolved spectroscopy (Ostensen and Telting, 2004).

Y.Lipkin,E.M.LeibowitzSchool of physics and Astronomy and the Wise Observatory, Raymondand Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, TelAviv, 69978, Israel; email: yiftah@wise.tau.ac.il, elia@wise.tau.ac.ilM. OrioIstituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico diTorino, Strada Osservatorio, 20, I-10025 Pino Torinese (TO), Italy;Department of Astronomy, 475 N. Charter Street, University ofWisconsin, Madison, WI 53706, USA; email:orio@cow.physics.wisc.eduAbstract. We conducted coordinated time-resolved observations of thelong-period Intermediate Polar V1062 Tauri at the WIYN Observatoryin Kitt Peak, Arizona and at Wise Observatory, Israel, and follow-upobservations at the second site. We confirm the presence of two previouslyreported periodicities of the system: a long period (P≈10 h) which wasinterpreted as the orbital period of the underlying binary system, and ashorter one (P≈ 1 h), which was interpreted as the spin of the white dwarf.Our data also revealed a third photometric periodicity, corresponding tothe orbital sideband of the spin period (OSB). The amplitude of the OSBwas found to be strongly correlated with the varying brightness of thesystem at the orbital cycle.Our observations suggest bi-modality in the photometric characteristicsof the star. In one mode, the light of V1062 Tau varies with the spinperiod of the white dwarf, whereas the OSB is undetected. In the othermode, the OSB is the main modulation at short time-scales, and the spinperiod is absent from the light curve. Switching between the two modesoccurred three times during the 10 weeks that spanned our observations.Also, we detected an outburst of 1.1 mag, which lasted between ∼1and∼5 days. In addition to the outburst, secular variations in the brightnessof the star (0.3 mag on time scale of a few of tens of days) suggest thatthe system was in a brief low state during 2002 January.1. IntroductionThe Intermediate Polar (IP) V1062 Tauri (H0459+246) was discovered as anX-ray source by Remillard et al. (1994). The optical counterpart was identifiedas a V ≈ 16 mag star with a strong UV flux. The optical spectrum of the star,as well as its X-ray and photometric features were characteristic of a magneticCV (Remillard et al. 1994).243

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt 5 (5.5 plus or minus 1.9) x 10(exp -15) ss(sup -1), after correcting for proper motion. Our results are consistent with previous P values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is |P||P(raised dot)|greater than or equal to 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for l=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set.

This article reports optical photometric and spectroscopic observations of the M33 star B416, establishing it as the fifth identified luminous blue variable (LBV) in that galaxy.

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is still unclear) interacts with material surrounding the site of the explosion. Observations of the evolving changes in emission at many wavelengths allow us to investigate the origin of the photons, and so potentially determine the nature of the explosion. Here we report the results of γ-ray, optical, infrared, submillimetre, millimetre and radio observations of the burst GRB990123 and its afterglow. Our interpretation of the data indicates that the initial and afterglow emissions are associated with three distinct regions in the fireball. The peak flux of the afterglow, one day after the burst, has a lower frequency than observed for other bursts; this explains the short-lived radio emission. We suggest that the differences between bursts reflect variations in the magnetic-field strength in the afterglow-emitting regions.

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding pre-white dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6 ± 0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 M☉. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dΠ/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors.

Continuous CCD photometry of Nova Aquilae 1995 was performed through the standard B, V, R and I filters during three nights in 1995 and with the I filter during 18 nights in 1996. The power spectrum of the 1996 data reveals three periodicities in the light curve: 0.2558 d, 0.06005 d and 0.079 d, with peak-to-peak amplitudes of about 0.012, 0.014 and 0.007 mag. respectively. The two shorter periods are absent from the power spectrum of the 1995 light curve, while the long one is probably already present in the light curve of that year. We propose that V1425 Aql should be classified as an Intermediate - Polar CV. Accordingly the three periods are interpreted as the orbital period of the underlying binary system, the spin period of the magnetic white dwarf and the beat period between them. Our results suggest that no later than 15 months after the outburst of the nova, accretion processes are taking place in this stellar system. Matter is being transferred from the cool component, most likely through an accretion disc and via accretion columns on to the magnetic poles of the hot component.

We observed V1974 Cygni with a CCD camera attached to the 1-m telescope of the Wise Observatory for 32 nights during the years 1994 and 1995. The two periodicities identified in the light curve, 0.0812585 day 1.95 hours and 0.085 d P..1 2.04 hr, are interpreted as the orbital period of the binary system and a superhump period, respectively. These results and the interpretation imply that no later than 2.5 years after the outburst of the nova an accretion disk is present in the nova stellar system. They also imply that the disk is in the state of “permanent superhump”. Using the precessing disk model of the superhump phenomenon and utilizing additional published observational results we estimate a mass of 0.75–1.07 M⊙ for the white dwarf in this nova system.

We analyze the last 125 years optical light curve of the symbiotic star AX Per through some remarkable correlations that we discovered in its power spectrum. The data were assembled from the literature and from the AAVSO database. A series of 6 major outbursts dominate the light curve. They are presented in the power spectrum as 13 harmonics of the fundamental frequency fa=1/Pa=1/23172 day-1. We refer to them as the "red" frequencies. Oscillations with the binary periodicity of the system Pb=1/fb=681.48 d are also seen in the light curve, with particularly large amplitudes during outbursts. The fb peak in the power spectrum is accompanied by 13 other peaks on each side, to which we refer as the "blue" frequencies. A distinct structure in the frequency distribution of the blue peaks, as well as in their peak power are best interpreted as reflecting beating of the 13 "red" frequencies with the binary one. We suggest, following others, that the major outbursts of the system result from events of intense mass loss from the giant star. Mass accretion onto the hot component, partially through the L1 point of the system, took place in the last 125 years at a rate that oscillated with the 13 first harmonics of the fa frequency. The binary orbit is slightly eccentric and periastron passages induced modulation of the L1 accretion at the binary frequency. Hence the fb oscillations in the brightness of the star of amplitude that is modulated by the "red" frequencies of the system., Accepted for publication in AJ

TAUVEX - Tel Aviv University UV Explorer is a space telescope that is currently being built in Israel, to be flown on board the Russian international sattelite SRG - Spectrum Roentgen Gamma, in late 1995 or early 1996. TAUVEX is an imager in the near UV spectral window. Its major goal is to make a survey of about 10% of the UV sky, in the range λ = 1350 - 3500A. A successful operation of TAUVEX will partially fill an important gap in our recognition of the sky, namely the distribution and the nature of the celestial UV sources, which are still mostly unknown. TAUVEX will also operate as a fast multicolor photometer in its UV range of operation. TAUVEX is aligned in parallel to the common optical axix of all the other instruments on board SRG, most of which are telescopes and monitors for high energy radiation. SRG will be thus able to perform for the first time in history simultaneous astronomical observations in one and the same celestial body, that cover together 7 order of magnitude of the recorded radiation. The observations of TAUVEX can be greatly enhanced by ground base observations.

This paper presents the results of high speed photo­ metric observations of the cool variable DA white dwarf (DAV) GD 154 obtained with the Whole Earth Telescope. GD 154 is one of the c;oolest pulsating DA white dwarfs and its study is important for understanding the red edge of the ZZ Ceti insta­ bility strip. Its power spectrum is dominated by three indepen­ dent modes (P 1 = 1186.5s, Pz = 1088.6s and P3 = 402.6s), and their harmonics and linear combinations. None of the half-integer harmonics reported in previous observations were present during the WET campaign. We propose that the ob­ served modes are trapped in the thin outer hydrogen layers. From the resulting identification of the pulsation modes, one derives an estimate of the rotation period (2.3 days) and of the mass ofthe outer hydrogen layer (2 X w-IO M *).

Researchers are sometimes faced with a set of observations that constitute a sparse coverage of a time series, suspected to be periodic. The period is unknown, but one can identify in the observed data a few points that are presumed to occur at the same phase, in different cycles of the unknown periodicity. We propose an algorithm that finds all periods which are compatible with such observed data, and suggest how to assess their statistical significance. The algorithm also provides stringent limits on the epochs of the fixed phase. We give three examples, from the field of astronomy, for application of our new algorithm. In the first one the algorithm reveals, on the basis of very few photometric observations, a highly significant period in the light curve of the recent classical Nova Herculis 1991. In the second example, in the series of arrival times of neutrinos from the supernova SN1987A, our algorithm yields a definite negative result. It proves that no significant exact periodicity is present in the data. In the third application, the algorithm provides new constraints on the epoch of one of the minima in the light curve of the stellar binary system 44i Bootis. We compare the method with other period search techniques, pointing out a few of its advantages, as well as some of its weaknesses.

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

Photometric observations of the nova V404 Cyg have been performed during 1989 May-July, immediately following its recent outburst. Measurements in the R photometric band have revealed periodic variability superposed on the general decline in the brightness of the star by 3 mag during that interval. The amplitude of the cyclic variation decrease from 0.24 mag in May-June, to 0.06 mag in July. A period analysis of the data suggests that the photometric cycle has a period of 3.0455 hr. This may well be the orbital period of the V404 Cyg binary system

(The Microlensing Planet Search Collaboration and the Wise Observatory GMAN Team)∗ Physics Department, University of Notre Dame, Notre Dame, IN 46530, USA† Departments of Physics and Astronomy, Univ. of Washington, Seattle, WA 98195, USA‡ School of Physics & Astronomy and Wise Obs., Tel-Aviv Univ.,Tel-Aviv 69978, Israelk Mt. Stromlo and Siding Spring Obs., Australian National Univ., Weston, ACT 2611, Australia# Department of Mathematics & Statistics, Monash Univ., Clayton, Victoria 3168, Australia∗∗ Department of Physics, University of California, Los Angeles, CA, USAThe study of extra-solar planetary systems has emerged as a new discipline of observational as-tronomy in the past few years with the discovery of a number of extra-solar planets.

Photometric observations of V4633 Sgr (Nova Sagittarii 1998) during 1998-2005 reveal the presence of a stable photometric periodicity at P1=180.8 min which is probably the orbital period of the underlying binary system. A second period was present in the light curve of the object for six years. Shortly after the nova eruption it was measured as P2=185.6 min. It has decreased monotonically in the following few years reaching the value P2=183.9 min in 2003. In 2004 it was no longer detectable. We suggest that the second periodicity is the spin of the magnetic white dwarf of this system that rotates nearly synchronously with the orbital revolution. According to our interpretation, the post-eruption evolution of Nova V4633 Sgr follows a track similar to the one taken by V1500 Cyg (Nova Cygni 1975) after that nova eruption, on a somewhat longer time scale. The asynchronism is probably the result of the nova outburst that lead to a considerable expansion of the white dwarf's photosphere. The increase in the moment of inertia of the star was associated with a corresponding decrease in its spin rate. Our observations have followed the spinning up of the white dwarf resulting from the contraction of its outer envelope as the star is slowly retuning to its pre-outburst state. It is thus the second known asynchronous polar classical nova., Comment: 6 pages, 8 figures. Accepted for publication in MNRAS

The recurrent nova RS Ophiuchi was observed five times with Chandra and XMM‐Newton, on four different dates after the 2006 outburst. In the first month we detected an emission spectrum with prominent lines of H and He‐like ions of metals including O, N, Si, Fe, Ne and Mg, showing a large range of plasma temperatures. At this stage the emission was originated in the red giant wind material shocked by the nova ejecta. Later spectra show a large increase in emission at longer wavelengths, peaking two months after the outburst. We attribute this to the emergence of the underlying white dwarf. Preliminary fits with NLTE white dwarf atmospheric models yield log g = 9, and T ∼800,000K, appropriate a white dwarf mass of >1.2 M⊙. The constant bolometric luminosity phase was very short, less than three months, as expected for a white dwarf mass ≈ 1.35 M⊙. We detected an oscillation with a period ∼35 s in March and April light curves, which may be due to the spin of the white dwarf and may represents further evidenc...

We construct a visual light curve of the symbiotic star MWC covering the last 87 years of its history. The data were assembled from the literature and from the AAVSO data bank. Most of the periodic components of the system brightness variation can be accounted for by the operation of three basic clocks of the periods P1 = 19,000 days, P2 = 1943 days, and P3 = 722 days. These periods can plausibly, and consistently with the observations, be attributed to three physical mechanisms in the system: the working of a solar-like magnetic dynamo cycle in the outer layers of the giant star of the system, the binary orbit cycle, and the sidereal rotation cycle of the giant star. MWC 560 is the seventh symbiotic star with historical light curves that reveal similar basic characteristics of the systems. The light curves of all these stars are well interpreted on the basis of the current understanding of the physical processes that are the major sources of the optical luminosity of these symbiotic systems.

We present a detailed analysis of the rapidly pulsating subdwarf B star (or EC14026 star) PG 1325+101. This analysis exploits the outcome of dedicated multisite observations using time-series photometry that revealed the presence of (at least) 15 periodicities in the luminosity modulation of this star (Silvotti et al. 2006, A&A, 459, 557, Paper I). Based on high-S/N, medium-resolution spectra obtained at the Steward Observatory's 2.3 m Telescope and at the 6.5 m MMT Telescope, new NLTE spectroscopic derivations of the atmospheric parameters of PG 1325+101 are presented. The mean values of T_eff = 35 050 ± 220 K, log g=5.81 ± 0.04, and log N(He)/N(H) = -1.70± 0.02 are in agreement with previous determinations, but are significantly more accurate. These improved spectroscopic parameters are essential for isolating a unique asteroseismic model solution for the observed pulsation spectrum of PG 1325+101. Using the "forward modeling" approach, our combined spectroscopic and asteroseismic analysis leads objectively to the identification of the (k, ?) indices of the 12 independent modes observed in this star, and to the determination of its structural parameters. The periods correspond to low-order acoustic modes with adjacent values of k and having degrees ?=0-4. They define a band of unstable modes, in agreement with nonadiabatic pulsation theory. The average dispersion achieved between the observed periods and the periods of the corresponding theoretical modes of the optimal model is only 0.46% (? 0.54 s), comparable to the results from similar analyses of other EC14026 stars analysed to date. The inferred structural parameters of PG 1325+101 are Teff= 35 050 ± 220 K, log g=5.811±0.004, log M{env}/M*=-4.18±0.10, M*=0.50±0.01 M? (i.e., close to the canonical mass of extreme horizontal branch stars), R/R?=0.145±0.002, and L/L?=28.3±1.5. In addition, by combining detailed model atmosphere calculations with V=14.019±0.012, we estimate that this star has an absolute visual magnitude M_V=4.45±0.04 and is located at a distance of d=820±21 pc. Finally, the presence of fine structure in the observed period spectrum suggests a rotation period of P=1.6±0.2 days, leading to an equatorial velocity of Veq=4.6±0.6 km s-1. Hence, asteroseismic evidence suggests that PG 1325+101 is a slow rotator, a conclusion reinforced by the limit V sin i < 20-30 km s-1, which we determined by modeling the He I 4471 line in our 1 Å-resolution MMT spectrum.

In this article we present the results of 215 h of time-series photometry on the rapidly pulsating subdwarf B star PG 1325+101 (T_eff = 35 000 K, log g = 5.8, log N(He)/N(H) = -1.7), obtained during 25 days of observation in Spring 2003 from nine different sites. As in previous observations, the temporal spectrum is dominated by the main peak at 7255.55 ?Hz, with an amplitude of about 2.7% which, however, is dropped to about 1.7% in February 2005. No secondary peaks close to the dominant pulsation mode are clearly detected. In addition, at least fourteen more pulsation frequencies are found: three of them at 7704.92, 9380.17 and 14511.10 ?Hz were already present in the discovery run with small differences in frequency, probably due to 1-day aliasing effects. The peak at 7704.92 ?Hz belongs to a triplet of almost equally spaced frequencies that could be due to rotational splitting and would imply a rotational period of about 1.6 days. Based on the results of this article, a detailed asteroseismic analysis of PG 1325+101 is presented in a separate paper (Charpinet et al. 2006b, A&A, 459, 565, Paper II).

We analyse a light curve of the symbiotic star BF Cyg, covering 114 years of its photometric history. The star had a major outburst around the year 1894. Since then the mean optical brightness of the system is in steady decline, reaching only in the last few years its pre-outburst value. Superposed on this general decline are some 6 less intense outbursts of 1-2 magnitude and duration of 2000-5000 days. We find a cycle of ~6376 days, or possibly twice this period, in the occurrence of these outbursts. We suggest that the origin of the system outbursts is in some magnetic cycle in the outer layers of the giant star of the system, akin to the less intense ~8000 days magnetic cycle of our Sun. We further find, that in addition to its well known binary period of 757.3 days, BF Cyg possesses also another photometric period of 798.8 days. This could be the rotation period of the giant star of the system. If it is, the beat period of these two periodicities, 14580 days, is the rotation period of a tidal wave on the surface of the giant. A 4th period of 4436 days, the beat period of the 14580 and the 6376 cycles is possibly also present in the LC. We predict that BF Cyg will be at the peak of its next outburst around the month of May in the year 2007. The newly discovered 798.8 days period explains the disappearance of the orbital modulation at some epochs in the light curve. The 757.3 oscillations will be damped again around the year 2013., 9 pages 7 figures. Accepted by MNRAS

International audience; We observed the pulsating hydrogen atmosphere white dwarf G 185-32 with the Whole Earth Telescope in 1992. We report on a weighted Fourier transform of the data detecting 18 periodicities in its light curve. Using the Hubble Space Telescope Faint Object Spectrograph time resolved spectroscopy, and the wavelength dependence of the relative amplitudes, we identify the spherical harmonic degree (l) for 14 pulsation signals. We also compare the determinations of effective temperature and surface gravity using the excited modes and atmospheric methods, obtaining Teff=11 960 ± 80 K, \log g=8.02 ± 0.04 and M=0.617 ± 0.024 M&sun;.

The pulsating DAV white dwarf G185-32 was observed with the Whole Earth Telescope in 1992, during XCov8. We briefly report on the weighted Fourier transform of the data obtained.

We report the discovery of a new eclipsing system less than one arcminute south of the pulsating DB white dwarf KUV 05134+2605. The object could be identified with the point source 2MASSJ0516288+260738 published by the Two Micron All Sky Survey. We present and discuss the first light curves as well as some additional colour and spectral information. The eclipse period of the system is 1.29d, and, assuming this to be identical to the orbital period, the best light curve solution yields a mass ratio of m2/m1=0.11, a radius ratio of r2/r1~1 and an inclination of 74 deg. The spectral anaylsis results in a Teff=4200K for the primary. On this basis, we suggest that the new system probably consists of a late K + Brown dwarf (which would imply a system considerably younger than ~0.01 Gyr to have r2/r1~1), and outline possible future observations., 15 pages, 9 figures, accepted for publication in Astronomy&Astrophysics. Revised version: optimized formatting only, no other changes

We present results from a multisite (‘Whole Earth Telescope’) photometric campaign on PG 1336−018, the close eclipsing binary system containing a pulsating subdwarf B (sdB) star. The main part of the campaign (1999 April) resulted in ~172 h of observations, representing a coverage of about 47 per cent, and additional data were obtained outside the core campaign. Periodogram analysis shows that the light variations are dominated by three frequencies near 5757, 5585 and 5369 μHz (~174, 179 and 186s, respectively), although many frequencies are present, particularly in the range 5000–6000 μHz (~200–170 s). We identify, with some confidence, 28 frequencies down to a semi-amplitude of 0.0005 in fractional intensity (equivalent to about 0.5 mmag). It is clear that the pulsation frequencies of PG 1336−018 have changed substantially since the 1996 discovery observations were made, and that amplitude changes occur, at least in the dominant three frequencies, on relatively short time-scales (of the order of a day). On the assumption that the pulsating star is phase-locked in the binary system, we have searched for rotational splitting of frequencies near the orbital and half of the orbital period, but the results are confused by aliasing at those frequencies (due to the data gaps caused by the eclipses). A preliminary model qualitatively matches the distribution of frequencies in PG 1336−018, with some good individual correspondences, but cannot be considered adequate because geometric cancellation should hide some of the modes which are apparently detected. Analysis of the pulsations during eclipse recovers three of the strongest modes, but the limited eclipse data – which can, at best, be only about 9 per cent of the total – do not allow mode identification at this stage. Simulations indicate that an overall coverage of about 80 per cent would be required for this to be viable. An attempt was made to determine phase shifts in the pulsation frequencies as a way of directly measuring the size of the binary orbit, but the uncertainties in the method are comparable to the light travel time across the orbit (probably less than a second).

The pulsating PG 1159 planetary nebula central star RXJ 2117+3412 has been observed over three successive seasons of a multisite photometric campaign. The asteroseismological analysis of the data, based on the 37 identified l = 1 modes among the 48 independent pulsation frequencies detected in the power spectrum, leads to the derivation of the rotational splitting, the period spacing and the mode trapping cycle and amplitude, from which a number of fundamental parameters can be deduced. The average rotation period is 1.16 ± 0.05 days. The trend for the rotational splitting to decrease with increasing periods is incompatible with a solid body rotation. The total mass is 0.56⁺⁰˙⁰²₋₀.₀₄ Mѳ and the He-rich envelope mass fraction is in the range 0.013–0.078 M*. The luminosity derived from asteroseismology is log(L/Lѳ) = 4.05⁺⁰˙²³₋₀.₃₂ and the distance 760⁺²³⁰₋₂₃₅ pc. At such a distance, the linear size of the planetary nebulae is 2.9 ± 0.9 pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed.

We report the analysis of 154 hours of nearly continuous high-speed photometric data on the pulsating DB white dwarf (DBV) GD 358 obtained during the Whole Earth Telescope (WET) run of May 1990. The Fourier transform (FT) of the light curve is dominated by power in the range from 1200 – 1700μHz with more than 180 significant peaks in the total transform. We also see significant power at the sums and differences of the dominant frequencies, indicating the importance of nonlinear behavior. We can use this data to obtain an accurate total stellar mass, and surface He layer mass. The implied surface He layer mass, if correct, provides a significant and surprising challenge to stellar evolution theory, as well as the theory of chemical mixing.

We report the results of observations of V4633 Sgr (Nova Sagittarii 1998) during 1998-2000. Two photometric periodicities were present in the light curve during the three years of observations: a stable one at P=3.014 h, which is probably the orbital period of the underlying binary system, and a second one of lower coherence, approximately 2.5 per cent longer than the former. The latter periodicity may be a permanent superhump, or alternatively, the spin period of the white dwarf in a nearly synchronous magnetic system. A third period, at P=5.06 d, corresponding to the beat between the two periods was probably present in 1999. Our results suggest that a process of mass transfer took place in the binary system since no later than two and a half months after the nova eruption. We derive an interstellar reddening of E(B-V)~0.21 from our spectroscopic measurements and published photometric data, and estimate a distance of d~9 kpc to this nova., Comment: 13 pages, latex, 7 figures, accepted for publication in MNRAS