We obtain new and precise information on the double white dwarf (DWD) population and on its gravitational-wave-driven merger rate, by combining the constraints on the DWD population from two previous radial-velocity-variation studies: One based on a sample of white dwarfs (WDs) from the Sloan Digital Sky Survey (SDSS, which with its low spectral resolution probes systems at separations a < 0.05 au), and one based on the ESO-VLT Supernova-Ia Progenitor surveY (SPY, which, with high spectral resolution, is sensitive to a < 4 au). From a joint likelihood analysis, the DWD fraction among WDs is f bin = 0.095 ± 0.020 (1σ, random) +0.010 (systematic) in the separation range 4 au. The index of a power-law distribution of initial WD separations (at the start of solely gravitational-wave-driven binary evolution), N (a) da ∝ a α da, is α = −1.30 ± 0.15 (1σ) +0.05 (systematic). The Galactic WD merger rate per WD is R merge = (9.7 ± 1.1) × 10 −12 yr −1 . Integrated over the Galaxy lifetime, this implies that 8.5 − 11 per cent of all WDs ever formed have merged with another WD. If most DWD mergers end as more-massive WDs, then some 10 per cent of WDs are DWD-merger products, consistent with the observed fraction of WDs in a "high-mass bump" in the WD mass function. The DWD merger rate is 4.5 − 7 times the Milky Way's specific Type-Ia supernova (SN Ia) rate. If most SN Ia explosions stem from the mergers of some DWDs (say, those with massive-enough binary components) then ∼ 15 per cent of all WD mergers must lead to a SN Ia.

From a sample of spectra of 439 white dwarfs (WDs) from the ESO-VLT Supernova-Ia Progenitor surveY (SPY), we measure the maximal changes in radial-velocity (∆RV max ) between epochs (generally two epochs, separated by up to 470 d), and model the observed ∆RV max statistics via Monte-Carlo simulations, to constrain the population characteristics of double WDs (DWDs). The DWD fraction among WDs is f bin = 0.100 ± 0.020 (1σ, random) +0.02 (systematic), in the separation range 4 AU within which the data are sensitive to binarity. Assuming the distribution of binary separation, a, is a power-law, dN/da ∝ a α , at the end of the last common-envelope phase and the start of solely gravitational-wavedriven binary evolution, the constraint by the data is α = −1.3 ± 0.2 (1σ) ±0.2 (systematic). If these parameters extend to small separations, the implied Galactic WD merger rate per unit stellar mass is R merge = (1 − 80) × 10 −13 yr −1 M −1 ⊙ (2σ), with a likelihood-weighted mean of R merge = (7 ± 2) × 10 −13 yr −1 M −1 ⊙ (1σ). The Milky Way's specific Type-Ia supernova (SN Ia) rate is likely R Ia ≈ 1.1 × 10 −13 yr −1 M −1 ⊙ and therefore, in terms of rates, a possibly small fraction of all merging DWDs (e.g. those with massive-enough primary WDs) could suffice to produce most or all SNe Ia.

WD 1145+017 is a unique white dwarf system that has a heavily polluted atmosphere, an infrared excess from a dust disk, numerous broad absorption lines from circumstellar gas, and changing transit features, likely from fragments of an actively disintegrating asteroid. Here, we present results from a large photometric and spectroscopic campaign with Hubble, Keck , VLT, Spitzer, and many other smaller telescopes from 2015 to 2018. Somewhat surprisingly, but consistent with previous observations in the u' band, the UV transit depths are always shallower than those in the optical. We develop a model that can quantitatively explain the observed "bluing" and the main findings are: I. the transiting objects, circumstellar gas, and white dwarf are all aligned along our line of sight; II. the transiting object is blocking a larger fraction of the circumstellar gas than of the white dwarf itself. Because most circumstellar lines are concentrated in the UV, the UV flux appears to be less blocked compared to the optical during a transit, leading to a shallower UV transit. This scenario is further supported by the strong anti-correlation between optical transit depth

We report the discovery of the sixth known eclipsing double white dwarf (WD) system, SDSS J1152+0248, with a 2.3968 ± 0.0003 h orbital period, in data from the Kepler Mission's K2 continuation. Analysing and modelling the K2 data together with ground-based fast photometry, spectroscopy, and radial-velocity measurements, we determine that the primary is a DA-type WD with mass M 1 = 0.47 ± 0.11 M , radius R 1 = 0.0197 ± 0.0035 R , and cooling age t 1 = 52 ± 36 Myr. No lines are detected, to within our sensitivity, from the secondary WD, but it is likely also of type DA. Its central surface brightness, as measured from the secondary eclipse, is 0.31 of the primary's surface brightness. Its mass, radius, and cooling age, respectively, are M 2 = 0.44 ± 0.09M , R 2 = 0.0223 +0.0064 −0.0050 R , and t 2 = 230 ± 100 Myr. SDSS J1152+0248 is a near twin of the double-lined eclipsing WD system CSS 41177.