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This paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterized as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light-curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar discs of these stars. The disc sizes are typically 3 – 6 times the areas of the central star. The disc temperatures are ∼40 per cent, and the disc luminosities are ∼10 per cent of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disc. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disc model for outbursting Be stars. [ABSTRACT FROM AUTHOR]

We present optical photometry of six intermediate polars that exhibit transitions to a low-flux state. For four of these systems, DW Cnc, V515 And, V1223 Sgr, and RX J2133.7+5107, we are able to perform timing analysis in and out of the low states. We find that, for DW Cnc and V515 And, the dominant periodicities in the light curves change as the flux decreases, indicating a change in the sources’ accretion properties as they transition to the low state. For V1223 Sgr, we find that the variability is almost completely quenched at the lowest flux, but we do not find evidence for a changing accretion geometry. For RX J2133.7+5107, the temporal properties do not change in the low state, but we do see a period of enhanced accretion that is coincident with increased variability on the beat frequency, which we do not associate with a change in the accretion mechanisms in the system. [ABSTRACT FROM AUTHOR]

Investigations of the formation of young stellar objects (YSOs) and planets require the detailed analysis of individual sources as well as statistical analysis of a larger number of objects. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) project provides such a unique opportunity by establishing a UV spectroscopic library of young high- and low-mass stars in the local universe. Here, we analyse optical photometry of the three ULLYSES targets (TX Ori, V505 Ori, V510 Ori) and other YSOs in the σ Ori cluster taken at the time of the HST observations to provide a reference for those spectra. We identify three populations of YSOs along the line of sight to σ Ori, separated in parallax and proper motion space. The ULLYSES targets show typical YSO behaviour with pronounced variability and mass accretion rates of the order of 10−8 M⊙ yr−1. Optical colours do not agree with standard interstellar reddening and suggest a significant contribution of scattered light. They are also amongst the most variable and strongest accretors in the cluster. V505 Ori shows variability with a 7 d period, indicating an inner disc warp at the co-rotation radius. Uncovering the exact nature of the ULLYSES targets will require improved detailed modelling of the HST spectra in the context of the available photometry, including scattered light contributions as well as non-standard reddening. [ABSTRACT FROM AUTHOR]

HO Puppis (HO Pup) was considered as a Be-star candidate based on its γ Cassiopeiae-type light curve, but lacked spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral-energy-distribution fit on broadband photometry, the Be-star nature of HO Pup is ruled out. Furthermore, based on the 28,700 photometric data points collected from various time-domain surveys and dedicated intensive-monitoring observations, the light curves of HO Pup closely resemble those of IW And-type stars (as pointed out by Kimura et al.), exhibiting characteristics such as a quasi-standstill phase, brightening, and dips. The light curve of HO Pup displays various variability timescales, including brightening cycles ranging from 23 to 61 days, variations with periods between 3.9 days and 50 minutes during the quasi-standstill phase, and a semiregular ∼14 day period for the dip events. We have also collected time-series spectra (with various spectral resolutions), in which Balmer emission lines and other spectral lines expected for an IW And-type star were detected (even though some of these lines were also expected to be present for Be stars). We detect Bowen fluorescence near the brightening phase, and that can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for four nights, the polarization variation was detected, indicating that HO Pup has significant intrinsic polarization. [ABSTRACT FROM AUTHOR]

The intermediate polar FO Aquarii (FO Aqr) experienced its first reported low-accretion states in 2016, 2017, and 2018, and, using newly available photographic plates, we identify prediscovery low states in 1965, 1966, and 1974. The primary focus of our analysis, however, is an extensive set of time-series photometry obtained between 2002 and 2018, with particularly intensive coverage of the 2016–2018 low states. After computing an updated spin ephemeris for the white dwarf (WD), we show that its spin period began to increase in 2014 after having spent 27 yr decreasing; no other intermediate polar has experienced a sign change of its period derivative, but FO Aqr has now done so twice. Our central finding is that the recent low states all occurred shortly after the WD began to spin down, even though no low states were reported in the preceding quarter-century, when it was spinning up. Additionally, the system's mode of accretion is extremely sensitive to the mass-transfer rate, with accretion being almost exclusively disk-fed when FO Aqr is brighter than V ∼ 14 and substantially stream-fed when it is not. Even in the low states, a grazing eclipse remains detectable, confirming the presence of a disklike structure (but not necessarily a Keplerian accretion disk). We relate these various observations to theoretical predictions that during the low state, the system's accretion disk dissipates into a non-Keplerian ring of diamagnetic blobs. Finally, a new XMM-Newton observation from a high state in 2017 reveals an anomalously soft X-ray spectrum and diminished X-ray luminosity compared to pre-2016 observations. [ABSTRACT FROM AUTHOR]

We present optical photometry of superoutbursts that occurred in 2016 of two WZ Sge-type dwarf novae (DNe), ASASSN-16dt and ASASSN-16hg. Their light curves showed a dip in brightness between the first plateau stage with no ordinary superhumps (or early superhumps) and the second plateau stage with ordinary superhumps. We find that the dip is produced by the slow evolution of the 3 : 1 resonance tidal instability and that it would likely be observed in low mass-ratio objects. An estimated mass ratio (q ≡ M2/M1) from the period of developing (stage A) superhumps [0.06420(3) d] was 0.036(2) in ASASSN-16dt. Additionally, its superoutburst has many properties similar to those in other low-q WZ Sge-type DNe: long-lasting stage-A superhumps, small superhump amplitudes, long delay of ordinary-superhump appearances, and a slow decline rate in the plateau stage with superhumps. Its very small mass ratio and observational characteristics suggest that this system is one of the best candidates for a period bouncer--a binary accounting for the missing population of post-period minimum cataclysmic variables. Although it is not clearly verified due to the lack of detection of stage-A superhumps, ASASSN-16hg might be a possible candidate for period bouncers on the basis of the morphology of its light curves and the small superhump amplitudes. Many outburst properties of period bouncer candidates would originate from the small tidal effects of their secondary stars. [ABSTRACT FROM AUTHOR]

Several tables are presented related to photometry of High-Amplitude Delta Scuti (HADS) stars, along with the information on star name, uncertainty of the epoch, and filter used for the observations.

The article discusses a study concerning the photometric observations of a Blazhko star. An overview on the comparison of differential photometry and the measurement in the magnitude offset between the two observers, is provided. Also emphasized is the Period04 software program that provides sine-wave fitting and least-squares fitting algorithms.

We report unfiltered photometry during superoutbursts of PU UMa in 2009 and 2012. The amplitude was 4.5 magnitudes above mean quiescence and lasted at least 9 to 10 days. Superhumps were present with a peak-to peak amplitude of up to ~0.3 mag, thereby confirming PU UMa to be a member of the SU UMa family of dwarf novae. The mean superhump period during the later part of the 2012 outburst was Psh = 0.08076(40)d. Analysis of the eclipse times of minimum, supplemented with data from other researchers, revealed an orbital period of Porb= 0.077880551(17)d. The superhump period excess was e= 0.037(5). During the 2012 outburst, which was the better observed of the two, the FWHM eclipse duration gradually declined from 9.5 to 5 min. The eclipse depth was up to 1.7 magnitudes. [ABSTRACT FROM AUTHOR]

NSV 1907, formerly listed as an irregular variable in variability catalogues, was classified as an Algol-type eclipsing binary in the Catalina Surveys Periodic Variable Star Catalogue. We have identified NSV 1907 as an ultraviolet (UV) bright source using measurements from the GALEX space telescope and detected obvious out-of-eclipse variability in archival photometric data from the Catalina Sky Survey, which instigated a closer examination of the object. A spectrum and extensive multicolour photometric observations were acquired, from which we deduce that NSV 1907 is a deeply eclipsing, nova-like cataclysmic variable. Apart from the orbital variations (deep eclipses with a period of P ≈ 6.63 hours), changes in mean brightness and irregular short-term variability (flickering) were observed. The presence of a secondary minimum at phase φ ≈ 0.5 was established, which indicates a significant contribution of the companion star to the optical flux of the system. We find possible evidence for sinusoidal variations with a period of P ≈ 4.2 d, which we interpret as the nodal precession period of the accretion disc. No outbursts or VY Scl-like drops in brightness were detected either by the CSS or during our photometric monitoring. Because of its spectral characteristics and the observed variability pattern, we propose NSV 1907 as a new moderately bright long-period SW Sextantis star. Further photometric and spectroscopic observations are encouraged. [ABSTRACT FROM AUTHOR]