I have a dream! If we could get enough long-period eclipsing binaries with multi-epoch spectroscopy, we could go a long way towards building a truly data-driven model of stellar spectra. It would be truly data-driven, because we would use the gravitational model of the eclipsing binary to get stellar masses and radii, and thus give good label (truth) inputs to a model like The Cannon for the stellar spectra. (Yes, if you have an eclipsing binary and spectroscopy for radial velocities, you get everything.) And then we could get densities, masses, and radii of stars for the interpretation of transit and radial-velocity results on exoplanets, without relying on stellar models. There are lots of other things to do too, like build population models for binary stars, and exploit the stellar models for Milky Way science. And etc.
Today, because of a meeting cancellation, both Adrian Price-Whelan and I got the full day off from responsibilities, so we decided to use it very irresponsibly. We searched the (very incomplete and under-studied) Kepler eclipsing binary list for binaries with long periods, deep eclipse depths, and APOGEE spectroscopy. It turns out there are lots! We started with the system KIC 9246715, which is a red-giant pair.
In the APOGEE spectrum, the pair of velocities (double line) is clearly visible, and it clearly changes from epoch to epoch. We found the velocities at each epoch first by auto-correlation and then by modeling the spectrum as a sum of two single stars. A project is born!