On the binary nature of dust-encircled BD+20 307

Three epochs of high-resolution spectra of the star BD +20 307 show that it is a short-period (similar to 3.4 day) spectroscopic binary of two nearly identical stars. Surprisingly, the two stars, although differing in effective temperature by only similar to 250 K and having a mass ratio of 0.91, show very different Li line equivalent widths. A Li 6707 angstrom line is detected from only the primary star, and it is weak. This star is therefore likely to be older than 1 Gyr. If so, the large amount of hot circumbinary dust must be from a very large and recent, but very late evolutionarily, collision of planetesimals.