  We present the results of a narrow-band infrared imaging survey of a narrow
strip (12' wide) around the galactic equator between 6 deg and 21 deg of
galactic longitude aimed at detecting field stars with strong CO absorption,
mainly late-type giants and supergiants. Our observations include follow-up low
resolution spectroscopy (R = 980) of 191 selected candidates in the H and K
bands. Most of these objects have photometric and spectroscopic characteristics
consistent with them being red giants, and some display broad, strong
absorption wings due to water vapor absorption between the H and K bands. We
also identify in our sample 18 good supergiant candidates characterized by
their lack of noticeable water absorption, strong CO bands in the H and K
windows, and HK_S photometry suggestive of high intrinsic luminosity and
extinctions reaching up to A_V ~40 mag. Another 9 additional candidates share
the same features except for weak H2O absorption, which is also observed among
some M supergiants in the solar neighbourhood. Interesting differences are
noticed when comparing our stars to a local sample of late-type giants and
supergiants, as well as to a sample of red giants in globular clusters of
moderately subsolar metallicity and to a sample of bulge stars. (...) We
propose that the systematic spectroscopic differences of our inner Galaxy stars
are due to their higher metallicities that cause deeper mixing in their
mantles, resulting in lower surface abundances of C and O and higher abundances
of CN, which contribute to the strength of the CaI and NaI features at low
resolution. Our results stress the limitations of using local stars as
templates for the study of composite cool stellar populations such as central
starbursts in galaxies (Abridged).
