摘要：We present 10 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 P2/P1 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 nonperiodic decreases of brightness of up to 0.5 mag over this 10 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 spectral energy distribution 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 be members of the rare group of giant stars with large mid-infrared excesses due to dust, objects which presumably have interesting evolutionary histories.
