# Anomalous Cepheids discovered in a sample of Galactic short period Type II Cepheids

## TL;DR

This study reanalyzed short period Type II Cepheids in the Galactic Field, identifying anomalous Cepheids and refining classifications, revealing that many previously classified BL Herculis stars are misclassified, and providing insights into their metallicity.

## Contribution

The paper presents a homogeneous Fourier analysis of Galactic short period Type II Cepheids, leading to reclassification and a better understanding of their metallicity properties.

## Key findings

- Identified 19 BL Her stars and 19 anomalous Cepheids in the sample.
- Discovered that over 50% of previously classified BL Her stars are misclassified.
- Found that anomalous Cepheids are generally metal-poor, while BL Her stars have Solar-like metallicity.

## Abstract

We revisited the short period Type II Cepheids (T2Cs), called the BL Herculis (BLHs), in the Galactic Field to derive a homogeneous analysis of their Fourier parameters. Only V-band data were compiled to make sure that it was directly comparable between the known variables of the OGLE-III catalogue and the 59 individual objects classified as short period Type II Cepheids in the General Catalogue of Variable Stars (GCVS) we had in our sample. The derived Fourier parameters were used to make the distinction between different classes of variables. From the 59 stars we found 19 BLHs, 19 fundamental mode Anomalous Cepheids (ACs) (8 of them were already known from the Catalina Sky Survey (CSS)), 1 first overtone AC, 2 were found to be possible peculiar W Virginis (pWVir), 11 classical Cepheids (DCEPs), and 7 stars were not pulsating variables at all. As a result we created a list of bright BLH stars in the Galactic Field, and separated the ACs, as well as other objects that were misclassified. The number of true BLHs decreased in our sample by more than 50%. We gathered the metallicity from spectroscopic measurements published in the literature. While the number of actual measurements is low, it is highly suggestive that ACs are metal poor. The mean metallicity from 8 measurements in 4 stars (UY Eri having 5 different [Fe/H] data points) is -1.12 dex, but if the higher value metallicity outliers of UY Eri are left out the mean metallicity becomes -1.88 dex, regardless if the AC is in the Milky Way itself or in a cluster. On the other hand, BLHs seem to have a Solar-like metallicity of 0 dex averaged from 21 measurements of 10 stars.