So where are all the outspiraling double WDs? Maybe they all merge, maybe we’re not detecting them, or something else altogether! For more, including eerily identical optical spectra and further discussion of evolutionary models, check out the paper arxiv.org/abs/2411.12796 (6/6)

Finally, there’s emerging tension b/w theory & data. MESA models of ELM+CO WDs predict the initial inspiral/period minimum should be >>faster than the subsequent outspiral. We should blindly detect |Pdot_in/Pdot_out|~4x more outgoing systems at 10 mins. That is… not seen. (5/6)

You can make this heuristic argument quantitative, and roughly constrain the allowed component masses just from measuring a P and P-dot. We get masses which look like a carbon-oxygen (CO) WD + an extremely-low mass (ELM) WD, like most known binaries at these periods. (4/6)

In two of our systems, we measure period derivatives–negative in one (GW emission), and positive in another. Positive P-dot is due to angular momentum (J) conservation: accreted matter loses J, so the orbit widens to compensate. It’s a tug-of-war between GWs & mass transfer (3/6)

Ultracompact binaries are the dominant source of mHz gravitational waves (GWs), and they bear heavily on space-based GW astronomy, binary evolution, and supernova progenitors. Personally, I’m most excited by their unique prospect of multi-messenger studies of accretion. (2/6)

Would you be willing to add me? Thanks for considering!