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tokens: We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. The authors would like to thank Alycia Weinberger for providing the published IRS spectrum of BD+20 307, and Rui Wang for the help in illustration preparation. This work is based on observations made with the Spitzer Space Telescope , which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This work was partially supported by contracts 1255094 and 1256424 from Caltech/JPL to the University of Arizona, and by NASA Grant NNX10AD38G. Facilities: AAVSO , Spitzer (IRAC)