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We describe the far-UV (1140-1740 Å) spectrum of the hydrogen-deficient R Coronae Borealis (RCB) star RY Sgr, obtained near maximum light (pulsational phase ≯0.1) by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope. The far-UV spectrum shows a photospheric continuum rising steeply toward longer wavelengths and two prominent emission features at the shorter wavelengths: C II λ1335 and Cl I λ1351 (the latter is radiatively fluoresced by the 10 times stronger C II multiplet). We also find evidence for CO A-X 4th-positive system absorption band heads and possible weak CO fluorescent emissions pumped by C II λ1335, but the inferred column densities are low (∼few times 1016 cm-2), consistent with formation in a warm (∼5000 K) atmospheric layer. The detection of CO molecules, if confirmed, would be significant, because they are thought to play a key role in the dust ejection episodes of RCB stars through the initiation of "molecular cooling catastrophes.".

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Astrophysical Journal

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