Volume 42 number 1 (2014)
We have used long-term AAVSO visual observations and Fourier and wavelet analysis to identify periods and to study long-term amplitude variations in forty-four red supergiants. Of these, twelve stars had data which were too sparse and/or had low amplitude and/or were without conspicuous peaks in the Fourier spectrum; six stars had only a long (2,500-4,000 days) period without significant amplitude variation. The other twenty-six stars had one or two periods, either "short" (hundreds of days) or "long" (thousands of days), whose amplitudes varied by up to a factor of 8, but more typically 2 to 4. The median timescale of the amplitude variation was 18 periods. We interpret the shorter periods as due to pulsation, and the longer periods as analogous to the "long secondary periods" found in pulsating red giants. We discuss possible explanations for the amplitude variations, including the effects of pulsation, rotation, convection cells, and stochastically-excited pulsations.