The unidentified 21 micron emission feature was first discovered using
the IRAS Low Resolution Spectra (Kwok, Volk, and Hrivnak 1989).  The sources
that show this emission feature have been limited to a small number of
carbon-rich stars which are in the post-asymptotic giant branch phase of
evolution.  While many suggestions have been made as to the origin of this
feature, there is no consensus on this question. However, there are
indications that the 21 micron feature may be related to the UIE features
which are usually attributed to PAH molecules.
Of the 4 sources where the feature was originally identified, IRAS 07134+1005
= SAO 96709 is the prototype because it has the strongest 21 micron feature
and is quite bright in the far-infrared.  This star is near the ISO sky hole
and therefore is not currently scheduled for any ISO observations.  It is
important to obtain ISO SWS01 and LWS01 observations of this source to allow
the identification of any features that accompany the 21 micron feature and
to allow detailed study of the continuum properties of whatever type of
dust carries the 21 micron feature.