Periodic Light Variability in Twelve Carbon-rich Proto-Planetary Nebulae

We present the results of a long-term (14 year) photometric monitoring program of 12 carbon-rich proto-planetary nebulae (PPNs). PPNs are objects in the evolutionary transition between the AGB and planetary nebula phases. These 12 have bright central stars (V=8-14 mag) with F G spectral types and faint nebulae (as seen with the HST). All of the objects show a periodicity in their light variations, although there is also evidence for multiple periods or small period changes. The pulsation periods range from 35 to 153 days, with the longer periods correlated with later spectral types. In fact, a tight correlation is seen between the period and effective temperature. The light variations range from 0.15 to 0.7 mag and are larger for the cooler objects.