  We present here near-infrared spectroscopy in the H and K bands of a
selection of nearly 80 stars that belong to various AGB types, namely S type, M
type and SR type. This sample also includes 16 Post-AGB (PAGB) stars. From
these spectra, we seek correlations between the equivalent widths of some
important spectral signatures and the infrared colors that are indicative of
mass loss. Repeated spectroscopic observations were made on some PAGB stars to
look for spectral variations. We also analyse archival SPITZER mid-infrared
spectra on a few PAGB stars to identify spectral features due to PAH molecules
providing confirmation of the advanced stage of their evolution. Further, we
model the SEDs of the stars (compiled from archival data) and compare
circumstellar dust parameters and mass loss rates in different types.
  Our near-infrared spectra show that in the case of M and S type stars, the
equivalent widths of the CO(3-0) band are moderately correlated with infrared
colors, suggesting a possible relationship with mass loss processes. A few PAGB
stars revealed short term variability in their spectra, indicating episodic
mass loss: the cooler stars showed in CO first overtone bands and the hotter
ones showed in HI Brackett lines. Our spectra on IRAS 19399+2312 suggest that
it is a transition object. From the SPITZER spectra, there seems to be a
dependence between the spectral type of the PAGB stars and the strength of the
PAH features. Modelling of SEDs showed among the M and PAGB stars that the
higher the mass loss rates, the higher the [K-12] colour in our sample.
