  H2O (22 GHz) and SiO masers (43, 86, 129 GHz) in the bipolar proto-planetary
nebula OH231.8+4.2 were simultaneously monitored using the 21-m antennas of the
Korean VLBI Network in 2009-2015. Both species exhibit periodic flux variations
that correlate with the central star's optical light curve, with a phase delay
of up to 0.15 for the maser flux variations with respect to the optical light
curve. The flux densities of SiO v = 2, J = 1-0 and H2O masers decrease with
time, implying that they may disappear in 10-20 years. However, there seems to
have been a transient episode of intense H2O maser emission around 2010. We
also found a systematic behaviour in the velocity profiles of these masers. The
velocities of the H2O maser components appear to be remarkably constant,
suggesting ballistic motion for the bipolar outflow in this nebula. On the
other hand, those of the SiO maser clumps show a systematic radial acceleration
of the individual clumps, converging to the outflow velocity of the H2O maser
clumps. Measuring the full widths at zero power of the detected lines, we
estimated the expansion velocities of the compact bipolar outflow traced by H2O
maser and SiO thermal line, and discussed the possibility of the expanding SiO
maser region in the equatorial direction. All of our analyses support that the
central host star of OH231.8 is close to the tip of the AGB phase, and that the
mass-loss rate recently started to decrease because of incipient post-AGB
evolution.
