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.

Abstract
1. INTRODUCTION
2. SOURCE SELECTION AND OBSERVATIONS
3. OBSERVATIONAL RESULTS
4. DESCRIPTIONS OF INDIVIDUAL OBJECTS
    4.1. OH13.1+5.1 (= IRAS 17528−1503)
    4.2. OH16.1−0.3 (= IRAS 18182−1504)
    4.3. OH38.10−0.13 (= IRAS 18588+0428)
    4.4. OH65.5+1.3 (= IRAS 19493+2905)
    4.5. IRAS 19312+1950
5. MASER AND INFRARED PROPERTIES ASSOCIATEDWITH A POST-AGB STAR EVOLUTION
6. SUMMARY
REFERENCES

• Jaeheon Kim(Shanghai Astronomical Observatory, Chinese Academy of Sciences) Corresponding author
• Se-Hyung Cho(Korea Astronomy and Space Science Institute)
• Dong-Hwan Yoon(Korea Astronomy and Space Science Institute/Seoul National University)