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This letter reports the first detection of a periodic light curve whose modulation is unambiguously due to rotation in a polluted white dwarf. TESS observations of WD 2138-332, at a distance of 16.1 pc, reveal a 0.39 per cent amplitude modulation with a 6.19 h period. While this rotation is relatively rapid for isolated white dwarfs, it falls within the range of spin periods common to those with detectable magnetic fields, where WD 2138-332 is notably both metal-rich and weakly magnetic. Within the local 20 pc volume of white dwarfs, multi-sector TESS data find no significant periodicities among the remaining 16 polluted objects (five of which are also magnetic), whereas six of 23 magnetic and metal-free targets have light curves consistent with rotation periods between 0.7 and 35 h (three of which are new discoveries). This indicates the variable light curve of WD 2138-332 is primarily a result of magnetism, as opposed to an inhomogeneous distribution of metals. From 13 magnetic and metallic degenerates with acceptable TESS data, a single detection of periodicity suggests that polluted white dwarfs are not rotating as rapidly as their magnetic counterparts, and planet ingestion is thus unlikely to be a significant channel for rapid rotation., Comment: 5 pages, 4 figures, and 2 tables, accepted by MNRAS Letters

This paper reports the ULTRACAM discovery of dipolar surface spots in two cool magnetic white dwarfs with Balmer emission lines, while a third system exhibits a single spot, similar to the prototype GD 356. The light curves are modeled with simple, circular, isothermal dark spots, yielding relatively large regions with minimum angular radii of 20 deg. For those stars with two light curve minima, the dual spots are likely observed at high inclination (or colatitude), however, identical and antipodal spots cannot simultaneously reproduce both the distinct minima depths and the phases of the light curve maxima. The amplitudes of the multi-band photometric variability reported here are all several times larger than that observed in the prototype GD 356; nevertheless, all DAHe stars with available data appear to have light curve amplitudes that increase toward the blue in correlated ratios. This behavior is consistent with cool spots that produce higher contrasts at shorter wavelengths, with remarkably similar spectral properties given the diversity of magnetic field strengths and rotation rates. These findings support the interpretation that some magnetic white dwarfs generate intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion. Spectroscopic time-series data for DAHe stars is paramount for further characterization, where it is important to obtain well-sampled data, and consider wavelength shifts, equivalent widths, and spectropolarimetry., Comment: 9 pages, 5 figures, 3 tables, accepted to MNRAS

White dwarfs stars are known to be polluted by their active planetary systems, but little attention has been paid to the accretion of wind from low-mass companions. The capture of stellar or substellar wind by white dwarfs is one of few methods available to astronomers which can assess mass-loss rates from unevolved stars and brown dwarfs, and the only known method to extract their chemical compositions. In this work, four white dwarfs with closely-orbiting, L-type brown dwarf companions are studied to place limits on the accretion of a substellar wind, with one case of a detection, and at an extremely non-solar abundance $m_{\rm Na}/m_{\rm Ca}>900$. The mass-loss rates and upper limits are tied to accretion in the white dwarfs, based on limiting cases for how the wind is captured, and compared with known cases of wind pollution from close M dwarf companions, which manifest in solar proportions between all elements detected. For wind captured in a Bondi-Hoyle flow, mass-loss limits $\dot M\lesssim 5\times10^{-17}$ M$_\odot$ yr$^{-1}$ are established for three L dwarfs, while for M dwarfs polluting their hosts, winds in the range $10^{-13} - 10^{-16}$ M$_\odot$ yr$^{-1}$ are found. The latter compares well with the $\dot M\sim 10^{-13} - 10^{-15}$ M$_\odot$ yr$^{-1}$ estimates obtained for nearby, isolated M dwarfs using Ly$\alpha$ to probe their astropsheres. These results demonstrate that white dwarfs are highly-sensitive stellar and substellar wind detectors, where further work on the actual captured wind flow is needed., Comment: 13 pages, 2 figures, 1 table, accepted to MNRAS

This study presents observations of two possible substellar survivors of post-main sequence engulfment, currently orbiting white dwarf stars. Infrared and optical spectroscopy of GD 1400 reveal a 9.98 h orbital period, where the benchmark brown dwarf has $M_2=68\pm8$ M$_{\rm Jup}$, $T_{\rm eff}\approx2100$ K, and a cooling age under 1 Gyr. A substellar mass in the lower range of allowed values is favoured by the gravitational redshift of the primary. Synthetic brown dwarf spectra are able to reproduce the observed CO bands, but lines below the bandhead are notably overpredicted. The known infrared excess towards PG 0010+281 is consistent with a substellar companion, yet no radial velocity or photometric variability is found despite extensive searches. Three independent stellar mass determinations all suggest enhanced mass loss associated with binary evolution, where the youngest total age for an isolated star is $7.5\pm2.5$ Gyr. A possible solution to this conundrum is the cannibalization of one or more giant planets, which enhanced mass loss post-main sequence, but were ultimately destroyed. PG 0010+281 is likely orbited by a debris disk that is comfortably exterior to the Roche limit, adding to the growing number of non-canonical disks orbiting white dwarfs. At present, only L-type (brown) dwarfs are known to survive direct engulfment during the post-main sequence, whereas T- and Y-type substellar companions persist at wide separations. These demographics indicate that roughly 50 M$_{\rm Jup}$ is required to robustly avoid post-main sequence annihilation, suggesting all closely-orbiting giant planets are consumed, which may contribute to mass loss and magnetic field generation in white dwarfs and their immediate progenitors., Comment: 15 pages, 8 figures, 3 tables. Accepted to MNRAS, dedicated to the memory of Tom Marsh

Many characteristics of dwarf carbon stars are broadly consistent with a binary origin, including mass transfer from an evolved companion. While the population overall appears to have old-disc or halo kinematics, roughly 2$\,$per cent of these stars exhibit H$\alpha$ emission, which in low-mass main-sequence stars is generally associated with rotation and relative youth. Its presence in an older population therefore suggests either irradiation or spin-up. This study presents time-series analyses of photometric and radial-velocity data for seven dwarf carbon stars with H$\alpha$ emission. All are shown to have photometric periods in the range 0.2--5.2$\,$d, and orbital periods of similar length, consistent with tidal synchronisation. It is hypothesised that dwarf carbon stars with emission lines are the result of close-binary evolution, indicating that low-mass, metal-weak or metal-poor stars can accrete substantial material prior to entering a common-envelope phase., Comment: 16 pages, 7 figures, Accepted to MNRAS

Despite thousands of spectroscopic detections, only four isolated white dwarfs exhibit Balmer emission lines. The temperature inversion mechanism is a puzzle over 30 years old that has defied conventional explanations. One hypothesis is a unipolar inductor that achieves surface heating via ohmic dissipation of a current loop between a conducting planet and a magnetic white dwarf. To investigate this model, new time-resolved spectroscopy, spectropolarimetry, and photometry of the prototype GD 356 are studied. The emission features vary in strength on the rotational period, but in anti-phase with the light curve, consistent with a cool surface spot beneath an optically thin chromosphere. Possible changes in the line profiles are observed at the same photometric phase, potentially suggesting modest evolution of the emission region, while the magnetic field varies by 10 per cent over a full rotation. These comprehensive data reveal neither changes to the photometric period, nor additional signals such as might be expected from an orbiting body. A closer examination of the unipolar inductor model finds points of potential failure: the observed rapid stellar rotation will inhibit current carriers due to the centrifugal force, there may be no supply of magnetospheric ions, and no anti-phase flux changes are expected from ohmic surface heating. Together with the highly similar properties of the four cool, emission-line white dwarfs, these facts indicate that the chromospheric emission is intrinsic. A tantalizing possibility is that intrinsic chromospheres may manifest in (magnetic) white dwarfs, and in distinct parts of the HR diagram based on structure and composition., Comment: 17 pages, 10 figures, 4 tables, accepted to MNRAS