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Stars with unusual properties can provide a wealth of information about rarestages of stellar evolution and exotic physics. However, determining the truenature of peculiar stars is often difficult. In this work, we conduct asystematic search for cool and luminous stars in the Magellanic Clouds withextreme variability, motivated by the properties of the unusual SMC star andThorne-\.Zytkow Object (T\.ZO) candidate HV2112. Using light curves fromASAS-SN we identify 38 stars with surface temperatures T $<$ 4800K,luminosities $\log$(L/L$_\odot$) $>$ 4.3, variability periods $>$ 400 days, andvariability amplitudes $\Delta$V $>$ 2.5 mag. Eleven of these stars possess thedistinctive double-peaked light curve morphology of HV2112. We use thepulsation properties and derived occurrence rates for these 12 objects toconstrain their nature. From comparisons to stellar populations and models, wefind that one star may be a red supergiant with large amplitude pulsations. Forthe other 11 stars we derive current masses of $\sim$5-10 M$_{\odot}$, belowthe theoretical minimum mass of $\sim$15 M$_{\odot}$ for T\.ZOs to be stable,casting doubt on this interpretation. Instead, we find that the temperatures,luminosities, mass-loss rates, and periods of these stars are consistent withpredictions for super-Asymptotic Giant Branch (s-AGB) stars that have beguncarbon burning but have not reached the superwind phase. We infer lifetimes inthis phase of $\sim($1$-$7) $\times$ 10$^{4}$ years, also consistent with ans-AGB interpretation. If confirmed, these objects would represent the firstidentified population of s-AGB stars, illuminating the transition between low-and high-mass stellar evolution.

astrophysical journal/the astrophysical journal

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from  ASAS-SN: Implications for Thorne-\.Zytkow Objects and Super-Asymptotic Giant  Branch Stars