  We have carried out a new photometric V,Rc study of 12 protoplanetary
nebulae, objects in the short-lived transition between the AGB and PN phases of
stellar evolution. These had been the subjects of an earlier study, using data
from 1994-2007, that found that all 12 varied periodically, with pulsation
periods in the range of ~38 to ~150 days. They are all carbon-rich, with F-G
spectral types. We combined our new (2008-2018) data with publicly-available
ASAS-SN data and determined new periods for their variability. The older and
newer period values were compared to investigate evidence of period change, for
which there is theoretical support that it might be detectable in a decade or
two in some cases. Such a detection is challenging since the light curves are
complicated, with multiple periods, changing amplitudes, and evidence of
shocks. Nevertheless, we found one, and possibly two, such cases, which are
associated with the higher temperature stars in the sample (7250 and 8000 K).
These results are most consistent with the evolution of stars at the lower end
of the mass range of carbon stars, ~1.5-2 M(sun). Several of the stars show
longer-term trends of increasing (six cases) or decreasing (one case)
brightness, which we think most likely due to changes in the circumstellar dust
opacity. There is one case of a possible ~1.8 yr period in addition to the
shorter pulsation. This is interpreted as possible evidence of an orbiting
companion.
