The O I] 1641A line as a probe of symbiotic star winds
  The neutral oxygen resonance 1302A line can, if the optical depth is
sufficiently high, de-excite by an intercombination transition at 1641A to a
metastable state. This has been noted in a number of previous studies but never
systematically investigated as a diagnostic of the neutral red giant wind in
symbiotic stars and symbiotic-like recurrent novae. We used archival $IUE$ high
resolution, and GHRS and STIS medium and high resolution, spectra to study a
sample of symbiotic stars. The integrated fluxes were measured, where possible,
for the O I 1302A and O I] 1641A lines. The intercombination 1641A line is
detected in a substantial number of symbiotic stars with optical depths that
give column densities comparable with direct eclipse measures (EG And) and the
evolution of the recurrent nova RS Oph 1985 in outburst. In four systems (EG
And, Z And, V1016 Cyg, and RR Tel), we find that the O I] variations are
strongly correlated with the optical light curve and outburst activity. This
transition can also be important for the study of a wide variety of sources in
which an ionization-bounded H II region is imbedded in an extensive neutral
medium, including active galactic nuclei, and not only for evaluations of
extinction.
