We present ten years of new photometric monitoring of the light variability of five evolved stars with strong mid-infrared emission from surrounding dust .
Three are known carbon-rich proto-planetary nebulae ( PPNe ) with F - G spectral types ; the nature of the other two was previously unknown .
For the three PPNe , we determine or refine the pulsation periods of IRAS 04296 + 3429 ( 71 days ) , 06530 - 0213 ( 80 days ) , and 23304+6147 ( 84 days ) .
A secondary period was found for each , with a period ratio P _ { 2 } / P _ { 1 } of 0.9 .
The light variations are small , 0.1 - 0.2 mag .
These are similar to values found in other PPNe .
The other two are found to be giant stars .
IRAS 09296+1159 pulsates with a period of only 47 days but reaches pulsational light variations of 0.5 mag .
Supplemental spectroscopy reveals the spectrum of a CH carbon star .
IRAS 08359 - 1644 is a G1 III star that does not display pulsational variability ; rather , it shows non-periodic decreases of brightness of up to 0.5 mag over this ten-year interval .
These drops in brightness are reminiscent of the light curves of R Corona Borealis variables , but with much smaller decreases in brightness , and are likely due to transient dust obscuration .
Its SED is very similar to that of the unusual oxygen-rich giant star HDE 233517 , which possesses mid-infrared hydrocarbon emission features .
These two non-PPNe turn out to members of the rare group of giant stars with large mid-infrared excesses due to dust , objects which presumably have interesting evolutionary histories .