Searches of the IRAS LRS database have revealed a small number of
 sources with an unidentified emission feature at 21 microns (Kwok,
 Volk, & Hrivnak 1989, ApJ, 387, L89).  Ground-based follow-up
 observations have found that this feature occurs only in F and G super-
 giants with large infrared excesses, and almost all 21 micron sources
 have been identified as stars in transition from the
 asymptotic giant branch to the planetary nebulae phase.

  Optical spectroscopy of the 21 micron sources have shown all of them
 to be carbon rich, and they often display strong 3.3, 3.4, and 11.3
 micron UIR emission features.  In order to identify the origin of the
 21 micron feature, it is necessary to obtain higher spectral resolution
 data to determine the fine structures of the feature.  We propose to
 use the SWS in the grating mode to observe 5 of the brightest 21 micron
 sources which have been discovered and observed by us from the ground.
 We also request observations of 6 other candidate 21 micron feature
 sources identified by UKIRT ground-based spectroscopy near 20 microns.