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Context.Accreting ultracompact white dwarf binaries contain a white dwarf that is accreting from a degenerate object. They have orbital periods shorter than 65 min. Aims.We report the discovery and the orbital period of four new eclipsing accreting ultracompact white dwarf binaries found using the Zwicky Transient Facility (ZTF) and discuss their photometric properties. Methods.We searched through a list of 4171 dwarf novae compiled using the ZTF and used the box least square method to search for periodic signals in the data. Results.We found four eclipsing accreting ultracompact binaries with orbital periods between 25.9 and 56 min. One had previously been published as an AM Canum Venaticorum (AM CVn), and the other three systems are new discoveries. The two shorter-period systems are likely also AM CVn systems, while the longest-period system, with a period of 56 min, showed multiple super-outbursts over two years, which is more consistent with it being a helium CV. 

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) atz = 0.4296 andz = 0.3103, respectively, which show an additional set of broad Mg IIabsorption lines, blueshifted by a few thousands kilometer second⁻¹with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frameg-band magnitude of SN 2020xga is −22.30 ± 0.04 mag and of SN 2022xgc is −21.97 ± 0.05 mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg IIline profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at ∼1.3 × 10¹⁶cm, moving with a maximum velocity of 4275 km s⁻¹, and the shell of SN 2022xgc is located at ∼0.8 × 10¹⁶cm, reaching up to 4400 km s⁻¹. These shells were expelled ∼11 and ∼5 months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of ∼7 − 9 M_⊙. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars > 50 M_⊙He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed. Modeling the spectral energy distribution of the host galaxy of SN 2020xga reveals a host mass of 10^7.8M_⊙, a star formation rate of 0.96_−0.26^+0.47M_⊙yr⁻¹, and a metallicity of ∼0.2 Z_⊙. 

ABSTRACT We present a sample of 14 hydrogen-rich superluminous supernovae (SLSNe II) from the Zwicky Transient Facility (ZTF) between 2018 and 2020. We include all classified SLSNe with peaks Mg < −20 mag with observed broad but not narrow Balmer emission, corresponding to roughly 20 per cent of all hydrogen-rich SLSNe in ZTF phase I. We examine the light curves and spectra of SLSNe II and attempt to constrain their power source using light-curve models. The brightest events are photometrically and spectroscopically similar to the prototypical SN 2008es, while others are found spectroscopically more reminiscent of non-superluminous SNe II, especially SNe II-L. 56Ni decay as the primary power source is ruled out. Light-curve models generally cannot distinguish between circumstellar interaction (CSI) and a magnetar central engine, but an excess of ultraviolet (UV) emission signifying CSI is seen in most of the SNe with UV data, at a wide range of photometric properties. Simultaneously, the broad H α profiles of the brightest SLSNe II can be explained through electron scattering in a symmetric circumstellar medium (CSM). In other SLSNe II without narrow lines, the CSM may be confined and wholly overrun by the ejecta. CSI, possibly involving mass lost in recent eruptions, is implied to be the dominant power source in most SLSNe II, and the diversity in properties is likely the result of different mass loss histories. Based on their radiated energy, an additional power source may be required for the brightest SLSNe II, however – possibly a central engine combined with CSI.