ES aquilae is an R Coronae Borealis star
ES Aq1, 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 Aq1 going back as far as 1893 using plates from the Harvard College Observatory as well as more recent photoelectric and visual observations. ES Aq1 displays the typical behavior of an RCB star, consisting of sharp declines at irregular intervals. The spectrum of ES Aq1 is also typical of a cool (Teff ∼ 5000 K) RGB star, showing strong absorption bands of C2 and CN, along with weak hydrogen and no detectable 13C. ES Aq1 is also an IRAS source indicating the presence of dust. Based on these data, we conclude that ES Aq1 is indeed an RCB star.