  Stellar evolution theory predicts that asymptotic giant branch stars undergo
a series of short thermal pulses that significantly change their luminosity and
mass on timescales of hundreds to thousands of years. Secular changes in these
stars resulting from thermal pulses can be detected as measurable changes in
period if the star is undergoing Mira pulsations. The American Association of
Variable Star Observers (AAVSO) International Database currently contains
visual data for over 1500 Mira variables. Light curves for these stars span
nearly a century in some cases, making it possible to study the secular
evolution of the pulsation behavior on these timescales. In this paper, we
present the results of our study of period change in 547 Mira variables using
data from the AAVSO. We find non-zero rates of period change, dlnP/dt, at the
2-sigma significance level in 57 of the 547 stars, at the 3-sigma level in 21
stars, and at the level of 6-sigma or greater in eight of the 547. The latter
eight stars have been previously noted in the literature, and our derived rates
of period changes largely agree with published values. The largest and most
statistically significant dlnP/dt are consistent with the rates of period
change expected during thermal pulses on the AGB. A number of other stars
exhibit non-monotonic period changes on decades-long timescales, the cause of
which is not yet known.
