Asteroseismology of the Red-giant Hosts KOI-3886 and iota Draconis

Kepler asteroseismology has played an important role in the characterization of host stars and their planetary systems. Target selection biases, however, meant that this synergy would remain mostly confined to main-sequence stars. The advent of TESS has since lifted this restriction, enabling the systematic search for transiting planets around seismic giants, as well as revisiting previously known evolved hosts using asteroseismology. Here, we present the detailed asteroseismic modeling of two high-luminosity red-giant branch hosts, KOI-3886 and iota Draconis. KOI-3886, observed by Kepler over 4 years and later by TESS over 1 sector, has been a longtime candidate host. iota Draconis, observed by TESS over 5 sectors, is known to host a planet in a highly eccentric orbit. The precise (~ 6%) seismic mass derived for iota Draconis was combined with new radial-velocity observations to detect an additional long-period companion. Regarding KOI-3886, asteroseismology was key in helping reveal the planet candidate as a false positive and reinterpreting the system as an eclipsing brown dwarf in a hierarchical triple with two evolved stars. This brings to light the importance of asteroseismology in the study of planetary orbital dynamics off the main sequence and its lesser known role in candidate vetting.