Variability in Protoplanetary Nebulae: VII. Light-Curve Studies of Five Medium-Bright, Oxygen-Rich, or Mixed-Chemistry Post-AGB/Post-RGB Objects

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We have monitored over a 10-year interval the light variations of five evolved stars with very large mid-infrared excesses. All five objects appear to have oxygen-rich or mixed oxygen-rich and carbon-rich chemistries. They all vary in light: four over a small range of ∼0.2 mag and the fifth over a larger range of ∼0.7 mag. Spectral types range from G2 to B0. Periodic pulsations are found for the first time in the three cooler ones, IRAS 18075-0924 (123 days), 19207+2023 (96 days), and 20136+1309 (142 days). No significant periodicity is found in the hotter ones, but they appear to vary on a shorter timescale of a few days or less. Two also show some evidence of longer-term periodic variations (∼4 yr). Three appear to be protoplanetary nebulae (PPNe), in the post-asymptotic giant branch (post-AGB) phase of stellar evolution. Their light variations are in general agreement with the relationships among temperature, pulsation period, and pulsation amplitude found in previously studied PPNe. The other two, however, appear to have too low a luminosity (1000-1500 L o˙), based on Gaia distances, to be in the post-AGB phase. Instead, they appear to be Milky Way analogs of the recently identified class of dusty post-red giant branch (post-RGB) stars found in the Magellanic Clouds, which likely had their evolution interrupted by interaction with a binary companion. If this is the case, then these would be among the first dusty post-RGB objects identified in the the Milky Way.