Trend Analysis of 51 Carbon Long-Period Variables

We have performed a trend analysis of 51 long-period variables (LPVs) of spectral types C, R, N, and S using 90 years of AAVSO data. We studied the periods and amplitudes, as well as the fall time (the time from maximum light to minimum), the rise time (from minimum to maximum), and the magnitudes at maximum and minimum. We also looked for time evolution of period and amplitude in the light curves themselves. The periods are more stable than the other parameters, with longer-period stars more likely to show period fluctuations than shorter-period stars. Fall time and rise time tend to evolve oppositely (mirror evolution), keeping the period fairly constant, whereas magnitude at maximum and minimum often evolve together (parallel evolution). About half of these stars are getting fainter, especially at maximum light, showing a secular dimming of magnitude at maximum; none shows a secular brightening.