# Type II Cepheids Pulsating in the First Overtone from the OGLE Survey

## TL;DR

This paper reports the discovery of the first known type II Cepheids pulsating solely in the first overtone, identified in the OGLE survey, and discusses their properties, rarity, and modeling challenges.

## Contribution

It presents the first identification of single-mode first-overtone BL Her stars and provides non-linear models explaining their light variations and mass-luminosity discrepancies.

## Key findings

- Two first-overtone BL Her stars discovered in OGLE data
- Models suggest these stars are more massive than typical BL Her stars
- Higher mass models conflict with observed luminosities

## Abstract

We report the discovery of the first type II Cepheids (BL Herculis stars) pulsating solely in the first overtone. We found two such objects among tens of millions of stars regularly observed by the OGLE survey in the Large Magellanic Cloud. Our classification and the pulsation mode identification is based on the position of these stars on the period-luminosity and color-magnitude diagrams and on the light curve analysis. We discuss why single-mode first-overtone BL Her pulsators must be very rare. For the two discovered stars we present non-linear models that successfully reproduce their light variation. These models indicate that both first-overtone pulsators should be more massive than it is typically assumed for BL Her stars, i.e. their masses should be above 0.75 Solar masses. However, the higher mass requires higher luminosity to match the observed periods of the stars, which is inconsistent with observations.