High-dispersion coude spectrograms have been obtained for all major phases of the RV Tauri variable AC Herculis. Model atmospheres have been constructed for each of these phases using line strengths measured from the plates. The model atmospheres and curve-of-growths analyses have both been used to determine that AC Her is metal poor. The star was found to be enhanced in carbon. AC Her was found to be surrounded by a substantial cloud of circumstellar gas where many of the molecular lines and some of the low excitation atomic lines originate. The circumstellar reddening of AC Her was found to be E(B-V) equals 0.35 plus or minus 0.05. Finally the evolutionary state of AC Her and other RV Tauri stars have been discussed in the light of this new information, and particular theoretical models suggested as possibilities.
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S. R. Baird (1981) studied this question.