2002PASP..114..846C

ES Aql, initially classified as a semiregular variable, is now believed to be a member of the R Coronae Borealis (RCB) class of stars, a small group of carbon-rich supergiants that undergo dramatic declines in brightness at irregular intervals. We present photometry of ES Aql going back as far as 1893 using plates from the Harvard College Observatory as well as more recent photoelectric and visual observations. ES Aql displays the typical behavior of an RCB star, consisting of sharp declines at irregular intervals. The spectrum of ES Aql is also typical of a cool (T_eff∼5000 K) RCB star, showing strong absorption bands of C₂ and CN, along with weak hydrogen and no detectable ¹³C. ES Aql is also an IRAS source indicating the presence of dust. Based on these data, we conclude that ES Aql is indeed an RCB star.