  The origin of the so-called "21 micron" feature which is especially prominent
in the spectra of some carbon-rich protoplanetary nebulae (PPNe}) is the matter
of a lively debate. A large number of potential band carriers have been
presented and discarded within the past decade. The present paper gives an
overview of the problems related to the hitherto proposed feature
identifications, including the recently suggested candidate carrier silicon
carbide. We also discuss the case for spectroscopically promising oxides.
  SiC is shown to produce a strong resonance band at 20-21 micron if coated by
a layer of silicon dioxide. At low temperatures, core-mantle particles composed
of SiC and amorphous SiO$_2$ indeed have their strongest spectral signature at
a position of 20.1 micron, which coincides with the position of the "21 micron"
emission band.
  The optical constants of another candidate carrier that has been relatively
neglected so far -- iron monoxide -- are proven to permit a fairly accurate
reproduction of the "21 micron" feature profile as well, especially when
low-temperature measurements of the infrared properties of FeO are taken into
account. As candidate carrier of the "21 micron" emission band, FeO has the
advantage of being stable against further oxidation and reduction only in a
narrow range of chemical and physical conditions, coinciding with the fact that
the feature, too, is detected in a small group of objects only. However, it is
unclear how FeO should form or survive particularly in carbon-rich PPNe.
