RY Sagittarii fades

2022 December 8

Jeremy Shears

RY Sgr is a yellow supergiant of the R Coronae Borealis type. As is typical for this class, its light curve is characterised by extended periods at maximum brightness (magnitude 6 to 7), during which there is occasionally a sudden fade to magnitude ~13 over a few weeks before a gradually brightening over the following several months. The interval between these episodes is irregular. The fading is due to the presence of dust clouds of carbon obscuring the star.

Variable Star Section observer John Toone reported RY Sgr fainter than normal, at visual magnitude 8.0, on Aug 29 and a BAA Alert email was issued by the Director to all members who have signed up for this service. The star actually brightened a little over the next few days, which may be related to its one-magnitude pulsations, which have a semi-regular period of ~38 days. A rapid fade then set in around Sep 7, reaching magnitude 13 in early October. At the time of writing (Oct 24), RY Sgr is still around magnitude 13. It will be interesting to discover how long the fade lasts. Observers will have to wait until after solar conjunction to find out.