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S Cephei

S Cephei is a highly evolved carbon-type Mira variable with a period of about 485 days.  S Cep exhibits the large amplitude pulsations typical of Miras, but there is more irregularity present in the light curve than usual.  The shape and amplitude of each cycle vary from one to the next, and there is perhaps a second period causing the traveling shoulder on the rising branch, visible clearly around JD 2454700.  A glance at the visual light curve also shows more magnitude scatter per unit time than average, which is also typical of highly reddened C-type stars.  This occurs more because of differences in observing technique and light sensitivity than anything physical in the star.  Like all Miras, S Cephei's days are numbered, and it won't be long -- cosmically speaking -- before it evolves off of the asymptotic giant branch, expelling its outer layers to become a white dwarf.

S Cep was one of many stars observed by Cox et al (2012) as part of a survey of AGB stars with the Herschel infrared space telescope.  Like many other evolved stars, S Cephei shows evidence of past mass loss in the form of infrared shells that surround the star out to very large radii -- in S Cep's case, about two tenths of a parsec (about 37,000 astronomical units). 

For more information visit: http://arxiv.org/abs/1110.5486
Prepared By: Matthew Templeton
AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484