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Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines.We report a stable 123.4-minute periodic variation in the strength and shape of the Ca II emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes 1 – 9 . One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf 10 – 14 , and it is expected that these objects will transition to helium accretion. These transitional CVs have been proposed as progenitors of helium CVs 13 – 18 . However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. Phase-resolved spectra, multi-band light curves and the broadband spectral energy distribution allow us to obtain precise and robust constraints on the masses, radii and temperatures of both components. Evolutionary modelling shows that ZTF J1813+4251 is destined to become a helium CV binary, reaching an orbital period under 20 minutes, rendering ZTF J1813+4251 a previously missing link between helium CV binaries and hydrogen-rich CVs. A 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a comparable temperature to that of the Sun but a 100 times greater density, accreting onto a white dwarf is reported.

Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original ‘black widow’, the eight-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20) 1 , high-energy emission originating from the pulsar 2 is irradiating and may eventually destroy 3 a low-mass companion. These systems are not only physical laboratories that reveal the interesting results of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars 4 , allowing for robust tests of the neutron star equation of state. Here we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate, the optical flux of which varies by a factor of more than ten. ZTF J1406+1222 pushes the boundaries of evolutionary models 5 , falling below the 80-minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low-metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic Centre, making it a probe of formation channels, neutron star kick physics 6 and binary evolution. ZTF J1406+1222 is a wide hierarchical triple system that hosts a low-metallicity subdwarf star and a ‘black widow’ millisecond pulsar that has a highly varying optical flux and a 62-minute period.

The discrepancy between abundances computed using optical recombination lines (ORLs) and collisionally excited lines (CELs) is a major, unresolved problem with significant implications for the determination of chemical abundances throughout the Universe. In planetary nebulae (PNe), the most common explanation for the discrepancy is that two different gas phases coexist: a hot component with standard metallicity, and a much colder plasma enhanced in heavy elements. This dual nature is not predicted by mass loss theories, and direct observational support for it is still weak. In this work, we present our recent findings that demonstrate that the largest abundance discrepancies are associated with close binary central stars. OSIRIS-GTC tunable filter imaging of the faint O ii ORLs and MUSE-VLT deep 2D spectrophotometry confirm that O ii ORL emission is more centrally concentrated than that of [Oiii] CELs and, therefore, that the abundance discrepancy may be closely linked to binary evolution.

We report here the results obtained from a systematic optical photometric survey aimed at finding new compact binary millisecond pulsars (also known as \"spiders\"): the COmpact BInary PULsar SEarch (COBIPULSE). We acquired multi-band optical images over one year around \\(33\\) unidentified Fermi-LAT sources, selected as pulsar candidates based on their curved GeV spectra and steady \\(\\gamma\\)-ray emission. We present the discovery of four optical variables coinciding with the Fermi sources 3FGL J0737.2\\(-\\)3233, 3FGL J2117.6\\(+\\)3725 (two systems in this field) and 3FGL J2221.6\\(+\\)6507, which we propose as new candidate spider systems. Indeed, they all show optical flux modulation consistent with orbital periods of \\(0.3548(5) \\ \\mathrm{d}\\), \\(0.25328(6) \\ \\mathrm{d}\\), \\(0.441961(2) \\ \\mathrm{d}\\), and \\(0.165(4) \\ \\mathrm{d}\\), respectively, with amplitudes \\(\\gtrsim 0.3 \\ \\mathrm{mag}\\) and colors compatible with companion star temperatures of \\(5000\\)--\\(6000 \\ \\mathrm{K}\\). These properties are consistent with the \"redback\" sub-class of spider pulsars. If confirmed as a millisecond pulsar, 3FGL J0737.2\\(-\\)3233 will be the closest known spider to Earth (\\(D=659_{-20}^{+16} \\ \\mathrm{pc}\\), from Gaia-DR3 parallax). We searched and did not find any X-ray sources matching our four candidates, placing \\(3\\sigma\\) upper limits of \\(\\sim10^{31}\\)--\\(10^{32} \\ \\mathrm{erg} \\ \\mathrm{s}^{-1}\\) (\\(0.3\\)--\\(10 \\ \\mathrm{keV}\\)) on their soft X-ray luminosities. We also present and discuss other multi-wavelength information on our spider candidates, from infrared to X-rays.

White dwarf photospheric parameters are usually obtained by means of spectroscopic or photometric analysis. These results are not always consistent with each other, with the published values often including just the statistical uncertainties. The differences are more dramatic for white dwarfs with helium-dominated photospheres, so to obtain realistic uncertainties we have analysed a sample of 13 of these white dwarfs, applying both techniques to up to three different spectroscopic and photometric data sets for each star. We found mean standard deviations of < \\(\\sigma T_{\\mathrm{eff}}\\) > = 524 K, < \\(\\sigma \\log g\\) > = 0.27 dex and < \\(\\sigma \\log(\\mathrm{H/He})\\) > = 0.31 dex for the effective temperature, surface gravity and relative hydrogen abundance, respectively, when modelling diverse spectroscopic data. The photometric fits provided mean standard deviations up to < \\(\\sigma T_{\\mathrm{eff}}\\) > = 1210 K and < \\(\\sigma \\log g\\) > = 0.13 dex. We suggest these values to be adopted as realistic lower limits to the published uncertainties in parameters derived from spectroscopic and photometric fits for white dwarfs with similar characteristics. In addition, we investigate the effect of fitting the observational data adopting three different photospheric chemical compositions. In general, pure helium model spectra result in larger \\(T_{\\mathrm{eff}}\\) compared to those derived from models with traces of hydrogen. The \\(\\log g\\) shows opposite trends: smaller spectroscopic values and larger photometric ones when compared to models with hydrogen. The addition of metals to the models also affects the derived atmospheric parameters, but a clear trend is not found.

We present a detailed study of the stellar and orbital parameters of the post-common envelope binary central star of the planetary nebula Ou~5. Low-resolution spectra obtained during the primary eclipse -- to our knowledge the first isolated spectra of the companion to a post-common-envelope planetary nebula central star -- were compared to catalogue spectra, indicating that the companion star is a late K- or early M-type dwarf. Simultaneous modelling of multi-band photometry and time-resolved radial velocity measurements was then used to independently determine the parameters of both stars as well as the orbital period and inclination. The modelling indicates that the companion star is low mass (\\(\\sim\\)0.25~M\\(_\\odot\\)) and has a radius significantly larger than would be expected for its mass. Furthermore, the effective temperature and surface gravity of nebular progenitor, as derived by the modelling, do not lie on single-star post-AGB evolutionary tracks, instead being more consistent with a post-RGB evolution. However, an accurate determination of the component masses is challenging. This is principally due to the uncertainty on the locus of the spectral lines generated by the irradiation of the companion's atmosphere by the hot primary (used to derive companion star's radial velocities), as well as the lack of radial velocities of the primary.

The photospheric metal pollution of white dwarfs is now well-established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still debated. Here, we report the analysis of GD424: a metal-polluted, helium-atmosphere white dwarf with a large amount of trace hydrogen. We determined the atmospheric parameters using a hybrid analysis that combines the sensitivity of spectroscopy to the atmospheric composition, \\(\\log(\\mathrm{H/He})\\), with that of photometry and astrometry to the effective temperature, \\(T_{\\mathrm{eff}}\\), and surface gravity, \\(\\log g\\). The resulting white dwarf mass, radius, and cooling age are \\(M_{\\mathrm{WD}}=0.77\\pm0.01\\,\\mathrm{M}_{\\odot}\\), \\(R_{\\mathrm{WD}}=0.0109\\pm0.0001\\,\\mathrm{R}_{\\odot}\\), and \\(\\tau_\\mathrm{cool}=215\\pm10\\) Myr, respectively. We identified and measured the abundances of 11 photospheric metals and argue that the accretion event is most likely either in the increasing or steady state, and that the disrupted planetesimal resembles either CI chondrites or the bulk Earth in terms of its composition. We suggest that the observed \\(1.33\\times 10^{22}\\) g of trace hydrogen in GD424 were at least partly acquired through accretion of water-rich planetary debris in an earlier accretion episode.

A large number of white dwarf and donor masses in cataclysmic variables have been found via modelling the primary eclipse, a method that relies on untested assumptions. Recent measurements of the mass of the white dwarf in the cataclysmic variable GY Cnc, obtained via modelling its ultraviolet spectrum, conflict with the mass obtained via modelling the eclipse light curve. Here we measure the radial velocity of the absorption lines from the donor star in GY Cnc to be \\(K_{\\rm abs} = 280 \\pm 2\\) kms\\(^{-1}\\), in excellent agreement with the prediction based on the masses derived from modelling the eclipse light curve. It is possible that the white dwarf mass derived from the ultraviolet spectrum of GY Cnc is affected by the difficulty of disentangling the white dwarf spectrum from the accretion disc spectrum.

We present MUSE deep integral-field unit spectroscopy of three planetary nebulae(PNe) with high abundance discrepancy factors (ADF > 20): NGC 6778, M 1-42 and Hf 2-2. We have constructed flux maps for more than 40 emission lines, and use them to build extinction, electron temperature (T\\(_e\\)), electron density (n\\(_e\\)), and ionic abundances maps of a number of ionic species. The effects of the contribution of recombination to the auroral [N II] and [O II] lines on T\\(_e\\) and the abundance maps of low-ionization species are evaluated using recombination diagnostics. As a result, low T\\(_e\\) values and a downward gradient of T\\(_e\\) are found toward the inner zones of each PN. Spatially, this nearly coincides with the increase of abundances of heavy elements measured using recombination lines in the inner regions of PNe, and strongly supports the presence of two distinct gas phases: a cold and metal-rich and a warm one with \"normal\" metal content. We have simultaneously constructed, for the first time, the ADF maps of O\\(^+\\) and O\\(^{2+}\\) and found that they centrally peak for all three PNe under study. We show that the main issue when trying to compute realistic abundances from either ORLs or CELs is to estimate the relative contribution of each gas component to the H I emission, and we present a method to evaluate it. It is also found that, for the studied high-ADF PNe, the amount of oxygen in the cold and warm regions is of the same order.