In Anderson & Lai 2018, we consider a new mechanism for generating spin-orbit misalignments in both hot and warm Jupiter systems, due to an inclined, external companion. The companion causes the hot Jupiter orbital axis to precess. Meanwhile, the oblate host precesses due to the presence of the hot Jupiter. As the host star spins down over Gyr timescales (due to magnetic braking), a spin-orbit resonance can occur when these two precession frequencies match. This can cause substantial teetering of the stellar spin axis, and the obliquity to grow a large value, in some cases to retrograde configurations.

We find that obliquity excitation is accompanied by a decrease in the mutual inclinations between the hot/warm Jupiter and the companion. This method of obliquity growth may therefore act to erase any previously higher inclinations in multi-planet systems.

Warm Jupiters have a high occurrence rate of external planetary companions with appropriate semi-major axes to cause obliquity excitation. As a result, warm Jupiters may be particular susceptible to resonant obliquity excitation, given high (20-30 degree) mutual inclinations between the planet orbits. At present, neither warm Jupiter obliquities nor mutual inclinations of warm Jupiters with companions have been extensively probed, and future observations are needed to resolve this issue. Resonant obliquity excitation is most effective for cool host stars which undergo significant spin-down.