CM Mic and other ER UMa stars showing standstills

We analyzed All-Sky Automated Survey for Supernovae (ASAS-SN), Asteroid Terrestrial-impact Last Alert System (ATLAS) and Transiting Exoplanet Survey Satellite (TESS) observations of CM Mic and found that this object belongs to a small group of ER UMa stars showing standstills. In addition to typical ER UMa-type cycles, the object showed standstills between 2017 and 2019 July, and in 2022. The supercycles varied between 49 and 83 d. In 2015, the object showed outbursts with a cycle length of ~35 d. An analysis of TESS observations during the 2020 July outburst detected superhumps with a mean period of 0.080251(6) d (value after the full development of superhumps). We also studied other ER UMa stars showing standstills mainly using Zwicky Transient Facility (ZTF) data. DDE 48, MGAB-V728 and ZTF18abmpkbj mostly showed ER UMa-type supercycles but showed one or two standstills. MGAB-V3488 was mostly in ER UMa states with short (~25 d) supercycles in 2020-2022 similar to RZ LMi. This object also showed long standstills. PS1-3PI J181732.65+101954.6 showed ER UMa-type supercycles up to 2020 May and entered a long standstill. ZTF18abncpgs showed standstills most of the time, but also showed ER UMa-type supercycles occasionally between standstills. ZTF19aarsljl is a likely member of this group. MGAB-V284 showed a pattern similar to ER UMa stars showing standstills but with a longer time-scale of normal outbursts. This object seems to be an ER UMa star with standstills above the period gap. None of the objects we studied showed a superoutburst arising from a long standstill, as recorded in NY Ser in 2018, although the 2019 June-July superoutburst of PS1-3PI J181732.65+101954.6 might have been an exception.Comment: 33 pages, 20 figures, VSOLJ Variable Star Bulletin No. 11

Spectra of V1405 Cas at the very beginning indicate a low-mass ONeMg white dwarf progenitor

The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarified despite its importance in the formation and evolution of WDs. We tackle this issue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which is an outlier given a combination of its very slow light-curve evolution and the recently reported neon-nova identification. We report its rapid spectral evolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and 81.90 hours after the discovery. The first spectrum is characterized by lines from highly-ionized species, most noticeably He II and N III. These lines are quickly replaced by lower-ionization lines, e.g., N II, Si II, and O I. In addition, Al II (6237 \r{A}) starts emerging as an emission line at the second epoch. We perform emission-line strength diagnostics, showing that the density and temperature quickly decrease toward later epochs. This behavior, together with the decreasing velocity seen in H$\alpha$, H$\beta$, and He I, indicates that the initial nova dynamics is reasonably well described by an expanding fireball on top of an expanding photosphere. Interestingly, the strengths of the N III and Al II indicate large abundance enhancement, pointing to an ONeMg WD progenitor as is consistent with its neon-nova classification. Given its low-mass nature inferred by the slow light-curve evolution and relatively narrow emission lines, it provides a challenge to the stellar evolution theory that predicts the lower limit of the ONeMg WD mass being $\sim$ 1.1 $M_\odot$.Comment: 15 pages, 5 figures, 2 tables, accepted to Ap