We present new light curves covering 14-19 years of observations of four bright proto-planetary nebulae (PPNs), all oxygen-rich and of F spectral type. They each display cyclical light curves with significant variations in amplitude. All four were previously 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 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 P 2 /P 1 ranging from 0.86 to 1.06. The variations in color reveal maximum variations in T eff of 400-770 K. These variations are due to pulsations in these post-asymptotic giant branch 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. 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.

No abstract

Carbon and oxygen isotopic ratios are reported for a sample of 51 SRb-and Lb-type variable asymptotic giant branch stars. Vibration-rotation first-and second-overtone CO lines in 1.5 to 2.5 µm spectra were measured to derive isotopic ratios for 12 C/ 13 C, 16 O/ 17 O, and 16 O/ 18 O. Comparisons with previous measurements for individual stars and with various samples of evolved stars, as available in the extant literature, are discussed. Using the oxygen isotopic ratios, the masses of the SRb stars can be derived. Combining the masses with Gaia luminosities, the SRb stars are shown to be antecedents of the Mira variables. The limiting parameters where plane-parallel, hydrostatic equilibrium model atmospheres can be used for abundance analysis of M giants are explored. Lebzelter et al.This is our second Paper In a series to study the isotopic ratios of C and O in AGB stars. In the first paper of this series (Hinkle et al. 2016, Paper I) we presented an extensive discussion on the influence of stellar parameters and evolution on the isotopic ratios of these two key elements measured at the stellar surface. Here we confine the discussion to a brief summary of the main effects and refer to Paper I (note in particular Figure 10), Lebzelter et al. (2015a), and references therein for a more extensive description.The CNO cycle depletes 12 C and enhances 13 C, a change that becomes visible as a steep drop in the 12 C/ 13 C ratio down to 10 to 25 after the first dredge up. A slight dependence of the resulting ratios on stellar mass is expected, with the more massive stars showing higher 12 C/ 13 C values (Paper I). Extensive observational evidence (e.g. Charbonnel et al. 1999) exists that extra-mixing along the red giant branch (RGB) can further reduce the ratio between the two carbon isotopes by a factor of two in low-mass stars. The interplay between production, mixing, and destruction of the various oxygen isotopes results in a dependence of 16 O/ 17 O on stellar mass (Boothroyd et al, 1994;El Eid 1994;Karakas & Lattanzio 2014), with the istopic ratio decreasing with increasing mass as different depths and thus different regions of 17 O enhancement are reached by the first dredge-up. 18 O, on the other hand, is depleted by H burning and therefore 16 O/ 18 O is increasing with the first dredge-up. This effect, however, is almost independent of mass.The second dredge-up has only a mild effect on the isotopic ratios on the surface. With the third dredge-up episodes, which are predicted to occur in stars with main-sequence masses between approximately 1.3 and 5 M at solar metallicity, 12 C/ 13 C is strongly enhanced, while there is little effect on the oxygen isotopic ratios. In the more massive AGB stars, Hot Bottom Burning (HBB) can effect the oxygen isotopes, in particular the abundance of 18 O which is significantly depleted (Lattanzio et al. 1996). As pointed out by Lebzelter et al. (2015a), the 16 O/ 18 O is good indicator of the primordial 18 O abundance in a star, so that star-to-star variations of this ratio also ...

We have monitored over a 10-year interval the light variations of five evolved stars with very large mid-infrared excesses. All five objects appear to have oxygen-rich or mixed oxygen-rich and carbon-rich chemistries. They all vary in light: four over a small range of ∼0.2 mag and the fifth over a larger range of ∼0.7 mag. Spectral types range from G2 to B0. Periodic pulsations are found for the first time in the three cooler ones, IRAS 18075–0924 (123 days), 19207+2023 (96 days), and 20136+1309 (142 days). No significant periodicity is found in the hotter ones, but they appear to vary on a shorter timescale of a few days or less. Two also show some evidence of longer-term periodic variations (∼4 yr). Three appear to be protoplanetary nebulae (PPNe), in the post-asymptotic giant branch (post-AGB) phase of stellar evolution. Their light variations are in general agreement with the relationships among temperature, pulsation period, and pulsation amplitude found in previously studied PPNe. The other two, however, appear to have too low a luminosity (1000–1500 L ⊙), based on Gaia distances, to be in the post-AGB phase. Instead, they appear to be Milky Way analogs of the recently identified class of dusty post-red giant branch (post-RGB) stars found in the Magellanic Clouds, which likely had their evolution interrupted by interaction with a binary companion. If this is the case, then these would be among the first dusty post-RGB objects identified in the the Milky Way.