High-resolution (R~70000) 4.6 micron Phoenix/Gemini spectra are
presented for a class of A-G post-AGB supergiants.  Infrared colors
of the program objects suggest the presence of a thick disk.  Optical
spectroscopy suggests that all these objects are single lined
spectroscopic binaries.  The spectra reveal for most stars the
presence of CO vibration-rotation fundamental lines and in some
stars H2O vibration-rotation lines.  The most extreme of these
objects is HR 4049 where the 4.6 micron spectrum shows a rich forest
of emission lines.  All the spectral lines observed are shown to
be circumbinary in origin.  The presence of H2O lines confirm the
oxygen-rich nature of the circumbinary gas which is in contrast to
previously detected carbon-rich dust in many of these objects.
Modeling of the line profiles suggests that the circumbinary gas
is located in a thin, rotating layer near the dust disk.  If the
inner diameter and inclination of the disk can be derived from other
information then the masses of the individual stars in the binary
can be derived.  In the cases analyzed so far the supergiant has
mass typical for a white dwarf, confirming its post-AGB nature.