We investigate the properties of AGB and post-AGB (PAGB) stars, planetary nebulae, and young stellar objects (YSOs) in our Galaxy through an analysis of observational data covering visual and infrared (IR) wavelengths. Utilizing datasets from IRAS, 2MASS, AllWISE, Gaia DR3, and the SIMBAD object database, we perform an in-depth comparison between observational data and theoretical models. For this comparison, we present various color-magnitude diagrams (CMDs) in visual and IR bands, as well as IR two-color diagrams (2CDs). Our results demonstrate that the CMDs, incorporating the latest distance and extinction data from Gaia DR3 for a majority of sample stars, are effective in distinguishing different classes of stars. To improve the precision of our analysis, we apply theoretical radiative transfer models for dust shells around AGB and PAGB stars. A thorough comparison of the theoretical models with observations across various IR 2CDs and CMDs shows a significant agreement. We find that AGB and PAGB stars are among the brightest classes in visual and IR bands. Furthermore, most YSOs are clearly distinguishable from AGB stars on various IR CMDs, exhibiting fainter absolute magnitudes in IR bands.

We present the results of simultaneous monitoring observations of H2O 61,6–52,3 (22GHz) and SiO J=1–0, 2–1, 3–2 maser lines (43, 86, 129GHz) toward five post-AGB (candidate) stars, using the 21-m single-dish telescopes of the Korean VLBI Network. Depending on the target objects, 7 – 11 epochs of data were obtained. We detected both H2O and SiO maser lines from four sources: OH16.1−0.3, OH38.10−0.13, OH65.5+1.3, and IRAS 19312+1950. We could not detect H2O maser emission toward OH13.1+5.1 between the late OH/IR and post-AGB stage. The detected H2O masers show typical double- peaked line profiles. The SiO masers from four sources, except IRAS 19312+1950, show the peaks around the stellar velocity as a single peak, whereas the SiO masers from IRAS 19312+1950 occur above the red peak of the H2O maser. We analyzed the properties of detected maser lines, and investigated their evolutionary state through comparison with the full widths at zero power. The distribution of observed target sources was also investigated in the IRAS two-color diagram in relation with the evolutionary stage of post-AGB stars. From our analyses, the evolutionary sequence of observed sources is suggested as OH65.5+1.3! OH13.1+5.1! OH16.1−0.3! OH38.10−0.13, except for IRAS 19312+1950. In addition, OH13.1+5.1 from which the H2O maser has not been detected is suggested to be on the gateway toward the post-AGB stage. With respect to the enigmatic object, IRAS 19312+1950, we could not clearly figure out its nature. To properly explain the unusual phenomena of SiO and H2O masers, it is essential to establish the relative locations and spatial distributions of two masers using VLBI technique. We also include the 1.2 – 160 μm spectral energy distribution using photometric data from the following surveys: 2MASS, WISE, MSX, IRAS, and AKARI (IRC and FIS). In addition, from the IRAS LRS spectra, we found that the depth of silicate absorption features shows significant variations depending on the evolutionary sequence, associated with the termination of AGB phase mass-loss.