A sample of sub-luminous type-II cepheids in OGLE and GAIA: indicators of gravo-nuclear loops?

Type-II Cepheids are pulsating variables that lie within the Cepheid instability strip of the H-R diagram. These are typically low-mass and metal-poor stars that are crossing the instability strip after evolution from the horizontal branch. These follow a period-luminosity law that is different from classical Cepheids and are about 2-3 magnitudes fainter for the same period. Surveys such as OGLE have vastly increased the number of type-II Cepheids. Additionally, the GAIA mission has provided parallax for a significant number of type-II Cepheids allowing for accurate P-L relations. Interestingly, a sample of sub-luminous sources is seen among the OGLE sample of type-II Cepheids. These sources seem to be systematically fainter in OGLE in Wassenhit index, the GAIA Gmag as well in corresponding 2MASS J, H and K. The errors from pulsation amplitude, photometric errors as well as parallax errors are not entirely sufficient to explain this deviation. In this work, we compare a sample of sub-luminous TIICs with that of a sample of expected luminosity. The samples are selected based on the quality of available parallax data. The metallicity shows almost identical characteristics. However, the sub-luminous sample is seen to have significantly higher specific gravity compared to the standard sample. One possible explanation for the lower luminosity of these sources is that these sources are going through a gravo-nuclear loop. We present the process of identifying these sources and discuss potential implications for the evolution of type-II Cepheids.