@article{21160, abstract = {Context. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels. Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems. Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models. Results. According to our measurements, the accretor’s effective temperature (Teff) is 23 300 ± 600 K and the surface gravity (log g) is 8.4 ± 0.3, which imply an accretor mass (MWD) of 0.86 ± 0.16 M⊙. We find a high nitrogen-to-carbon abundance ratio by mass of > 153. Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels.}, author = {Yu, W. and Pala, A. F. and Kupfer, T. and Gänsicke, B. T. and Koester, D. and Belloni, D. and Wong, T. L.S. and Schreiber, M. R. and van Roestel, Joannes C and Brown, A. J. and Waagen, E. O. and González-Carballo, J. L. and Bednarz, S. and Bernacki, K. and De Martino, D. and Fernández Mañanes, E. and González Farfán, R. and Green, M. J. and Groot, P. J. and Hambsch, F. J. and Knigge, C. and Martin-Velasco, J. L. and Morales-Aimar, M. and Myers, G. and Naves Nogues, R. and Poggiani, R. and Popowicz, A. and Ramsay, G. and Reina-Lorenz, E. and Rodríguez-Gil, P. and Salto-González, J. L. and Sion, E. M. and Steeghs, D. and Szkody, P. and Toloza, O. and Tovmassian, G.}, issn = {1432-0746}, journal = {Astronomy and Astrophysics}, publisher = {EDP Sciences}, title = {{The evolutionary history of ultra-compact accreting binaries: I. Chemical abundances and the formation channel of the eclipsing AM CVn system ZTF J225237.05-051917.4 from HST spectroscopy}}, doi = {10.1051/0004-6361/202557568}, volume = {706}, year = {2026}, } @article{21705, abstract = {We report the discovery of ATLAS J101342.5−451656.8 (hereafter ATLAS J1013−4516), an 8.56 minute orbital-period mass-transferring AM Canum Venaticorum (AM CVn) binary with a mean Gaia magnitude of G = 19.51, identified via periodic variability in light curves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) of Gaia white dwarf candidates. Follow-up with the Large Lenslet Array Magellan Spectrograph shows a helium-dominated accretion disk, and high-speed ULTRACAM photometry reveals pronounced primary and secondary eclipses. We construct a decade-long timing baseline leveraging light curves from the ATLAS and Gaia surveys, as well as the high-speed imagers ULTRACAM on the New Energy Telescope and proto-Lightspeed on the Magellan Clay telescope. From this timing baseline, we measure an orbital period derivative of P 1.60 0.07 10 = ± × 12 s s−1. Interpreted in the context of stable mass transfer, the magnitude and sign of P indicate that the orbital evolution is governed by the interplay between gravitationalwave-driven angular-momentum losses and mass transfer, directly probing the donor’s structural response to mass loss. We constrain the accretor and donor mass based on stable mass-transfer arguments assuming angularmomentum loss dominated by gravitational-wave emission, allowing us to infer the characteristic gravitational wave strain of the binary for future space-based GW observatories such as the Laser Interferometer Space Antenna (LISA). We predict a characteristic strain corresponding to a 4 yr LISA signal-to-noise ratio ≳10, establishing ATLAS J1013−4516 as a strong prospective LISA source that will probe long-term orbital evolution in the mass-transferring regime.}, author = {Chickles, Emma T. and Chakraborty, Joheen and Burdge, Kevin B. and Dhillon, Vik S. and Draghis, Paul and El-Badry, Kareem and Green, Matthew J. and Householder, Aaron and Hughes, Sarah and Layden, Christopher and Littlefair, Stuart P. and Munday, James and Pelisoli, Ingrid and Redden, Maya S. and Tonry, John and van Roestel, Joannes C and Angile, Francesco Elio and Brown, Alex J. and Segura, Noel Castro and Dinsmore, Jack and Dyer, Martin and Furesz, Gabor and Gabutti, Michelle and Garbutt, James and García-Mejía, Juliana and Jarvis, Daniel and Kennedy, Mark R. and Kerry, Paul and Mccormac, James and Mo, Geoffrey and Osip, Dave and Parsons, Steven and Pike, Eleanor and Piotrowski, John J. and Romani, Roger W. and Sahman, David and Simcoe, Rob}, issn = {1538-4357}, journal = {The Astrophysical Journal}, number = {2}, publisher = {IOP Publishing}, title = {{An eclipsing 8.56 minutes orbital period mass-transferring binary}}, doi = {10.3847/1538-4357/ae4871}, volume = {1000}, year = {2026}, } @article{21780, abstract = {It is predicted that half or more of all cataclysmic variables (CVs) should have evolved past the period minimum and now exist as so-called period bouncers where a white dwarf should be accreting from a Roche lobe filling substellar companion. However, this prediction stands in stark contrast to observations, where only a few per cent of CVs are found in this evolutionary phase. A potential solution to this discrepancy is that a magnetic field emerges from within the white dwarf after the system has reached the period minimum. The transfer of angular momentum from the spin of the white dwarf into the orbit then pushes the two stars apart, detaching them for potentially billions of years. Here we present the discovery of ZTF J021804.16+071152.93, a detached 0.69 +- 0.01 M⁠, 19 MG magnetic white dwarf plus 37 +- 5MJup brown dwarf binary with an orbital period of 1.7 h. The kinematics of the system indicate that it is a high probability member of the Galactic thick disc. However, this strongly disagrees with the much younger age of the system obtained from the white dwarf parameters, implying that the system may have been accreting in the past. This system is therefore consistent with having detached as a result of the emergence of the magnetic field of the white dwarf when the system was still mass transferring, and may represent the ultimate fate for many (perhaps even most) CVs.}, author = {Parsons, S. G. and Brown, A. J. and Casewell, S. L. and Littlefair, S. P. and van Roestel, Joannes C and Rebassa-Mansergas, A. and Murillo-Ojeda, R. and Zorotovic, M. and Schreiber, M. R. and Bagnulo, S. and Stroet, M. A. and Castro Segura, N. and Dhillon, V. S. and Dyer, M. J. and Garbutt, J. A. and Green, M. J. and Jarvis, D. and Kennedy, M. R. and Kerry, P. and Mccormac, J. and Munday, J. and Pelisoli, I. and Pike, E. and Sahman, D. I. and Yates, A.}, issn = {1365-2966}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford University Press}, title = {{ZTF J021804.16+071152.93: A dead cataclysmic variable and potential solution to the missing period bouncer cataclysmic variables}}, doi = {10.1093/mnras/stag521}, volume = {547}, year = {2026}, } @article{21842, abstract = {AM CVn stars are ultra-compact semi-detached binaries consisting of a white dwarf primary and a hydrogen-depleted secondary. In this paper, we present spectroscopic and photometric results of 15 transient sources pre-classified as AM CVn candidates. Our analysis confirms 9 systems of the type AM CVn, 3 hydrogen-rich cataclysmic variables (accreting white dwarfs with near-main-sequence stars for donors), and 3 systems that could be evolved cataclysmic variables. Eight of the AM CVn stars are analysed spectroscopically for the first time, which increases the number of spectroscopically confirmed AM CVns by about 10%. TESS data revealed the orbital period of the AM CVn star ASASSN-20pv to be Porb =27.282 min, which helps to constrain the possible values of its mass ratio. TESS also helped to determine the superhump periods of one AM CVn star (ASASSN-19ct, Psh =30.94 min) and two cataclysmic variables we classify as WZ Sge stars (Psh =90.77 min for ZTF18aaaasnn and Psh =91.6min for ASASSN-15na).We identified very different abundances in the spectra of theAM CVns binaries ASASSN-15kf and ASASSN-20pv (both Porb ∼27.5min), suggesting different type of donors. Six of the studied AMCVns are X-ray sources, which helped to determine their mass accretion rates. Photometry shows that the duration of all the superoutbursts detected in the AM CVns is consistent with expectations from the disc instability model. Finally, we provide refined criteria for the identification of new systems using all-sky surveys such as LSST.}, author = {Kára, Jan and Rivera Sandoval, Liliana and Mendoza, Wendy and Maccarone, Thomas and Pichardo Marcano, Manuel and Salazar Manzano, Luis E. and Oelkers, Ryan J. and van Roestel, Joannes C}, issn = {1448-6083}, journal = {Publications of the Astronomical Society of Australia}, publisher = {Cambridge University Press}, title = {{A study of transients from ground-based surveys reveals new ultra-compact accreting white dwarf binaries}}, doi = {10.1017/pasa.2026.10184}, volume = {43}, year = {2026}, } @article{21897, abstract = {Ultracompact binary systems, consisting of two compact objects in an orbit $\lesssim 0.5 {\rm R}_\odot$, should exhibit measurable rates of orbital period change ($\dot{P} \ne 0$) due to the emission of gravitational waves (GWs). Measurements of $\dot{P}$ have so far been limited to the shortest-period ultracompact binaries ($\lesssim 20$  min). Among the AM CVn-type subclass, several works have proposed the presence of extra angular momentum loss beyond GW emission, with magnetic braking being a widely discussed mechanism. If present, this magnetic braking would dominate the angular momentum loss of AM CVn-type binaries with orbital periods $\gtrsim 30$ min. In this work, we present a long-term eclipse timing study of two AM CVn-type binaries, YZ LMi and Gaia14aae, with respective orbital periods of 28.3 min and 49.7 min and continuous observations since 2006 and 2015. Both systems show $\dot{P}$ consistent with zero within $2\sigma$. Their $3\sigma$ upper limits are $1.1 \times 10^{-13}\, {\rm s \, s}^{-1}$ and $9.7 \times 10^{-14}\, {\rm s \, s}^{-1}$, respectively. These non-detections are most simply explained by a scenario in which secular angular momentum loss is not substantially stronger than GW emission at all orbital periods, but is combined with deviations from the secular $\dot{P}$ whose time-scales span decades but whose amplitude is $\lesssim 10^{-13}\, {\rm s \, s}^{-1}$. Our non-detections of $\dot{P}$ represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission.}, author = {Green, Matthew J and Marsh, Thomas R and van Roestel, Joannes C and Wong, Tin Long Sunny and Belloni, Diogo and Kilic, Mukremin and Breedt, Elmé and Brown, Alex and Copperwheat, Chris M and Chakpor, Anurak and Dhillon, V S and Segura, Noel Castro and Dyer, Martin J and Garbutt, James and Jarvis, Dan and Kengkriangkrai, Vasu and Kennedy, Mark R and Kerry, Paul and Kupfer, Thomas and Littlefair, S P and McCormac, James and Munday, James and Parsons, Steven G and Pike, Eleanor and Pelisoli, Ingrid and Rodríguez-Gil, Pablo and Sahman, David I and Yates, Amalie}, issn = {1365-2966}, journal = {Monthly Notices of the Royal Astronomical Society}, keywords = {binaries: close – stars, dwarf novae – novae, cataclysmic variables – white dwarfs}, number = {3}, publisher = {Oxford University Press}, title = {{No period change in two long-period AM CVn binaries}}, doi = {10.1093/mnras/stag673}, volume = {548}, year = {2026}, }