Volume 43 number 2 (2015)
We have used up to 12 decades of AAVSO visual observations, and the AAVSO VSTAR software package to determine new and/or improved periods of 5 pulsating biperiodic carbon (C-type) red giants, and 12 pulsating biperiodic oxygen (M-type) red giants. We have also determined improved periods for 43 additional C-type red giants, in part to search for more biperiodic C-type stars, and also for 46 M-type red giants. For a small sample of the biperiodic C-type and M-type stars, we have used wavelet analysis to determine the time scales of the cycles of amplitude increase and decrease. The C-type and M-type stars do not differ significantly in their period ratios (first overtone to fundamental). There is a marginal difference in the lengths of their amplitude cycles. The most important result of this study is that, because of the semiregularity of these stars, and the presence of alias, harmonic, and spurious periods, the periods which we and others derive for these stars—especially the smaller-amplitude ones—must be determined and interpreted with great care and caution. For instance: spurious periods of a year can produce an apparent excess of stars, at that period, in the period distribution.