We present new light curves covering 14 to 19 years of observations of four bright proto-planetary nebulae (PPNs), all O-rich and of F spectral type. They each display cyclical light curves with significant variations in amplitude. All four were prev
iously known to vary in light. Our data were combined with published data and searched for periodicity. The results are as follows: IRAS 19475+3119 (HD 331319; 41.0 days), 17436+5003 (HD 161796; 45.2 days), 19386+0155 (101.8 days), and 18095+2704 (113.3 days). The two longer periods are in agreement with previous studies while the two shorter periods each reveal for the first time reveal a dominant period over these long observing intervals. Multiple periods were also found for each object. The secondary periods were all close to the dominant periods, with P2/P1 ranging from 0.86 to 1.06. The variations in color reveal maximum variations in T(eff) of 400 to 770 K. These variations are due to pulsations in these post-AGB objects. Maximum seasonal light variations are all less than 0.23 mag (V), consistent for their temperatures and periods with the results of Hrivnak et al. (2010) for 12 C-rich PPNs. For all of these PPNs, there is an inverse relationship between period and temperature; however, there is a suggestion that the period-temperature relationship may be somewhat steeper for the O-rich than for the C-rich PPNs.
We have investigated the light variability in a sample of 22 carbon-rich post-AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), based primarily on photometric data from the OGLE survey. All are found to vary. Dominant pe
riods are found in eight of them; these periods range from 49 to 157 days, and most of these stars have F spectral types. These eight are found to be similar to the Milky Way Galaxy (MWG) carbon-rich proto-planetary nebulae (PPNs) in several ways: (a) they are in the same period range of ~38 to ~160 days, (b) they have similar spectral types, (c) they are (all but one) redder when fainter, (d) they have multiple periods, closely spaced in time, with a average ratio of secondary to primary period of ~1.0, and as an ensemble, (e) they show a trend of decreasing period with increasing temperature, and (f) they show a trend of decreasing amplitude with decreasing period. However, they possibly differ in that the decreasing trend of period with temperature may be slightly offset from that of the MWG. These eight are classified as PPNs. The other 14 all show evidence of variability on shorter timescales. They are likely hotter PPNs or young planetary nebulae. However, in the MWG the numbers of PPNs peak in the F-G spectral types, while it appears that in the LMC they peak at a hotter B spectral type. One of the periodic ones shows a small, R Coronae Borealis-type light curve drop.
