Volume 50 number 1 (2022)
SRd variables are semiregular pulsating variable giants or supergiants of spectral type F, G, or K. But why are they not regular? This paper presents a detailed study, using light curve analysis, and Fourier and wavelet analysis, of data from the All-Sky Automated Survey for Supernovae (ASAS-SN), on 37 arbitrarily-selected SRd variables to examine the possible causes of their non-regularity. Of the 37 variables, 30 showed significant variations in pulsation amplitude, 11 showed significant “wandering” of the period, 6 showed abrupt period shifts, 7 showed a possible long secondary period (LSP), 8 showed possible bimodal pulsation, and 4 showed otherwise complicated behavior. Variable pulsation amplitude is therefore the most common of several phenomena which lead to their non-regularity. It also occurs in RV Tauri variables and pulsating red giants, but its physical cause is not known, nor is the cause of period wandering. Because there was some previous evidence that LSPs were rare among SRd variables, 13 SRd stars with the longest ASAS-SN periods were similarly analyzed. That analysis, and examination of the light curves of several dozen other SRd variables with long ASAS-SN periods showed clearly that LSPs are common in SRd variables. In longer-period SRd variables in the ASAS-SN variable star catalog, the catalog period is usually the LSP, rather than the pulsation period. LSPs in RV Tauri variables and in red giants have been ascribed to binarity; that may be the case in SRd variables also. A dozen W Virginis variables and 30 RV Tauri variables were also analyzed to study the overlap and possible relationship between CW, RV, and SRd stars. There is considerable overlap between these types.