**7. Related Work**

Two related fields of study not ye<sup>t</sup> discussed are the effects of stellar pulsation and binarity in RSG/YSG stars. Stellar pulsations may serve as an effective observational discriminant between post-RSG stars and main-sequence OB-stars migrating towards the RSG branch for the first time. Saio et al. [82] modeled the pulsation periods of supergiants with the Geneva stellar evolution code and found that most non-radial pulsations are only excited after significant mass loss on the RSG branch. In particular, the models appear to predict the pulsation periods of the *α* Cygni variables, suggesting these stars are in a He-burning, post-RSG phase. Further, the models presented in that work and its follow-up [83] sugges<sup>t</sup> that CNO surface abundances should be different in the two stellar populations, owing to increased dredge up along the RSG branch. Convective cells and their effect on both surface abundance and pulsational properties remains largely unexplored on the observational side of stellar astrophysics for post-RSG stars but may well be a useful diagnostic for evolutionary state.

One final topic to consider is the role of binarity in massive star systems. More than 70% of O- and B-type stars have a binary companion [107,108] suggesting that their evolved counterparts—typically RSG+B binary pairs—should also be numerous. Recent observations by Neugent et al. [109] show that optical spectroscopy alone can be used to detect RSG+B star binaries and studying these systems prior to a SN explosion is critical, since Kochanek et al. [110] demonstrates that only ∼5% of SN remnants contain a surviving star plus remnant binary. Eldridge et al. [111] sugges<sup>t</sup> that binary interactions may well be the cause of SN Type Ibc explosions, as the interaction strips much of the hydrogen from the stellar surface before the terminal explosion. However, the modeling from that work does not necessarily implicate the higher mass RSG/YSG pairs in the statistics for Type II-P progenitor systems. That said, earlier work by Eldridge et al. [5] sugges<sup>t</sup> that enhanced mass transfer, colliding winds, gravitational distortion, and other binary interaction effects may indeed hasten an RSG towards core-collapse. The field of binary interactions is an entire genre of astrophysics which we cannot hope to summarize in this work but we note that binarity can significantly alter the HR diagram for high-mass/high-luminosity objects. A grea<sup>t</sup> summary of binarity and multiplicity in stellar systems as it relates to RSGs in particular can be found in Chapter 5 in Levesque [112].

Many of the luminous warm and cool hypergiants have extensive CS ejecta and evidence for high mass-loss events. The yellow hypergiants and many of the yellow supergiants are candidates for post red supergiant evolution. IRC +10420, Var A and the extreme red supergiant VY CMa may be the special cases that provide the clues to understanding evolution near the top of the HR Diagram. These stars represent short-lived, unstable states that signal the last stages in RSG evolution and the brief post-RSG transition as the star returns to warmer temperatures. This class of post-RSG stars with complex mass-loss histories may be the missing piece on the HR Diagram and the solution to the red supergiant problem.

**Funding:** Support for this research was funded in part by the Universities Space Research Association (USRA) for the author's post-doctoral research position at the SOFIA Science Center.

**Conflicts of Interest:** The authors declare no conflict of interest.
