Maser surveys have shown that many potential OH/IR stars have no OH
 masers in their CS envelopes, despite modest requirements which should
 be implicitly met by IRAS colour-selected candidates. It is probable
 that these `OH/IR colour mimics' are related to symbiotic stars and
 have a degenerate companion (hidden in the optical and near-IR by
 circumstellar dust) which disrupts the masing action. Here we propose
 to compare far-IR line emission from colour mimics with that from
 symbiotic stars. The circumstellar dust shells will be much less
 opaque in the far-IR and, since a hot companion would inevitably
 stimulate line emission, our spectra will constitute a simple and
 effective test of whether hot stars are present. Line intensities and
 ratios for symbiotics (and mimics, if lines are detected) will be used
 to analyse abundances and ionization conditions. The temperature of
 any colour mimic companions will help to tie down the mechanism (be it
 radiative or gravitational) by which masers are suppressed in
 symbiotic Miras. Our sample contains examples of colour mimics with a
 range of IRAS colours and LRS types, as well as symbiotics with AGB
 giants and first-ascent giants. These have been chosen to avoid
 duplication of the symbiotic and OH/IR stars in the Core Programme.