Yesterday was a slow day; today by contrast we have a new mass-constraint, this by the most direct means possible namely gravitational microlensing. It affects white dwarfs.
The nearest six are Sirius B, Procyon B, van Maanen, LP 145-141 = LAWD 37, 40 Eridani B (boo!) and Stein 2051 B.
Johan Stein, SJ, got to name his star-system on account he seems the first to notice it as a double-star. Before that, I think, it got confused with Gliese/GJ 169. By that token, hey, it was close to at least one other background star, which occultation was recorded in March 2014; doi 10.1126/science.aal2879.
Direct measurement was and is hard to do with a binary. Hence why Sirius B hasn't been so measured to my knowledge; at 20 AU system semimajor it's probably too close to A most the time. That goes double for 40 Eridani classically confused with omicron-Eridani. These get measured by Kepler. Stein 2051 is contrarily a wide system whose motions relative to each other exceed a millennium, thus making Keplerian analysis impractical since identification in 1908. Kind of like Proxima versus Alpha Centauri; we didn't even know they were bound until the last decade or so.
It also helps, I further imagine, that white dwarfs are so small for their temperature, meaning their luminosity is low, so they don't interfere with the gravity-bump. Wide-angle brown dwarfs should be even easier.
LAWD 37 by itself and owning no close-in planets cannot be measured by Kepler... at all. Luckily the direct measurement has now constrained its mass: to 0.56 M.
By this token it should add to our knowledge of white dwarf evolution - this one blew up 1.25 Gya so many galactic spins ago, so certainly formed in some faraway cluster not many My before that (as a giant). This is data to add for such dwarfs as are not yet measured so directly, starting with those closer three and like 40 Eri B.
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