  Several classes of pulsating stars are now known to undergo slow changes in
amplitude; these include pulsating red giants and supergiants, and yellow
supergiants. We have used visual observations from the AAVSO International
Database, and wavelet analysis of 39 red giants, 7 red supergiants, and 3
yellow supergiants, to test the hypothesis that an increase in amplitude would
result in an increase in period, because of non-linear effects in the
pulsation. For most of the stars, the results are complex and/or indeterminate,
due to the limitations of the data, the small amplitude or amplitude variation,
or other processes such as random cycle-to-cycle period fluctuations. For the
dozen stars which have substantial amplitude variation, and reasonably simple
behavior, there is a 75-80% tendency to show a positive correlation between
amplitude and period.
