common-envelope binary stars

[Research barely proceeds during this pandemic. Don't take these posts to be evidence of a lot of research activity here.]

Mathieu Renzo (Flatiron) showed some calculations in Stars & Exoplanets meeting (now virtual) about how common-envelope stars might appear in LISA. The idea is that when stars with compact cores in binary systems orbitally decay, they hit a point at which they must merge into a single star. These sources might be in the LISA band for a while. My loyal reader knows that Adrian Price-Whelan (Flatiron) and I have found some very short-period binary companions in radial-velocity data from APOGEE; some too short-period to be orbiting outside their host stars! We have presumed that these signals are mis-classified asteroseismic oscillations. However, maybe they could be common-envelope? Renzo pointed out that there should be interesting spectral signatures if they are common-envelope. Let's check!