RY Sagittarii

RY Sagittarii is an orange supergiant and an R Coronae Borealis type variable star in the constellation Sagittarius. Although it ostensibly has the spectrum of a G-type star, it differs markedly from most in that it has almost no hydrogen and a lot of carbon. The star is so remote that its parallax, distance and hence luminosity were impossible to calculate with any accuracy. The Hipparcos satellite calculated its parallax at 1.29 milliarcseconds, yielding a distance of 1,716.6 light-years (526.32 parsecs) from Earth. Its effective temperature has been calculated at 7250 K.

It is one of the three brightest R Coronae Borealis stars visible to Earth observers, along with R Coronae Borealis and V854 Centauri. It is also a pulsating variable, with a semiregular period of 38 days. Its light curve has been studied for over a hundred years and is typical for the class, characterised by a sudden drop in brightness of several magnitudes over a few weeks before gradually brightening over the following several months. The timing between these dimmings is irregular. The cause of the drop in magnitude is the presence of dust clouds of carbon obscuring (and most likely ejected from) the star, though the mechanism how this might occur is not known. Extensive clouds have been detected with ESO's Very Large Telescope Interferometer.