White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

We present Hubble Space Telescope (HST) ultraviolet spectroscopy of the white dwarfs PG 0843+516, PG 1015+161, SDSS 1228+1040, and GALEX 1931+0117, which accrete circumstellar planetary debris formed from the destruction of asteroids. Combined with optical data, a minimum of five and a maximum of 11 different metals are detected in their photospheres. With metal sinking time‐scales of only a few days, these stars are in accretion/diffusion equilibrium, and the photospheric abundances closely reflect those of the circumstellar material. We find C/Si ratios that are consistent with that of the bulk Earth, corroborating the rocky nature of the debris. Their C/O values are also very similar to those of bulk Earth, implying that the planetary debris is dominated by Mg and Fe silicates. The abundances found for the debris at the four white dwarfs show substantial diversity, comparable at least to that seen across different meteorite classes in the Solar system. PG 0843+516 exhibits significant overabundances of Fe and Ni, as well as of S and Cr, which suggests the accretion of material that has undergone melting, and possibly differentiation. PG 1015+161 stands out by having the lowest Si abundance relative to all other detected elements. The Al/Ca ratio determined for the planetary debris around different white dwarfs is remarkably similar. This is analogous to the nearly constant abundance ratio of these two refractory lithophile elements found among most bodies in the Solar system. Based on the detection of all major elements of the circumstellar debris, we calculate accretion rates of ≃1.7 × 108  to ≃1.5 × 109 g s−1. Finally, we detect additional circumstellar absorption in the Si iv 1394, 1403 Å doublet in PG 0843+516 and SDSS 1228+1040, reminiscent to similar high‐ionization lines seen in the HST spectra of white dwarfs in cataclysmic variables. We suspect that these lines originate in hot gas close to the white dwarf, well within the sublimation radius.

Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution. We searched for such planets in NN Ser ab, an eclipsing short-period binary that shows long-term eclipse time variations. Using published, reanalysed, and new mid-eclipse times of NN Ser ab obtained between 1988 and 2010, we find excellent agreement with the light-travel-time effect produced by two additional bodies superposed on the linear ephemeris of the binary. Our multi-parameter fits accompanied by N-body simulations yield a best fit for the objects NN Ser (ab)c and d locked in the 2:1 mean motion resonance, with orbital periods P c 15.5 yrs and P d 7.7 yrs, masses M c sin i c 6.9 M Jup and M d sin i d 2.2 M Jup , and eccentricities e c 0 and e d 0.20. A secondary χ 2 minimum corresponds to an alternative solution with a period ratio of 5:2. We estimate that the progenitor binary consisted of an A star with ∼2 M and the present M dwarf secondary at an orbital separation of ∼1.5 AU. The survival of two planets through the common-envelope phase that created the present white dwarf requires fine tuning between the gravitational force and the drag force experienced by them in the expanding envelope. The alternative is a second-generation origin in a circumbinary disk created at the end of this phase. In that case, the planets would be extremely young with ages not exceeding the cooling age of the white dwarf of 10 6 yrs.

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.