# Ultralow-amplitude RR Lyrae Stars in M4

## TL;DR

This paper reports the discovery of ultralow-amplitude RR Lyrae-like stars in the globular cluster M4, revealing a new class of variable stars with amplitudes much smaller than typical RR Lyrae stars, which could provide insights into stellar physics.

## Contribution

The study identifies and characterizes a new class of millimagnitude RR Lyrae stars with extremely low amplitudes, expanding understanding of stellar variability near the instability strip.

## Key findings

- Discovery of two ultralow-amplitude RR Lyrae candidates in M4.
- Amplitudes are about 200 times smaller than typical RR Lyrae stars.
- Stars are located just outside the instability strip, one blueward and one redward.

## Abstract

We report evidence for a new class of variable star, which we dub millimagnitude RR Lyrae (mmRR). From K2 observations of the globular cluster M4, we find that out of 24 horizontal branch stars not previously known to be RR Lyrae variables, two show photometric variability with periods and shapes consistent with those of first overtone RR Lyrae variables. The variability of these two stars, however, have amplitudes of only one part in a thousand, which is ${\sim}$200 times smaller than for any RR Lyrae variable in the cluster, and much smaller than any known RR Lyrae variable generally. The periods and amplitudes are: 0.33190704 d with 1.0 mmag amplitude and 0.31673414 d with 0.3 mmag amplitude. The stars lie just outside the instability strip, one blueward and one redward. The star redward of the instability strip also exhibits significant multi-periodic variability at lower frequencies. We examine potential blend scenarios and argue that they are all either physically implausible or highly improbable. Stars such as these are likely to shed valuable light on many aspects of stellar physics, including the mechanism(s) that set amplitudes of RR Lyrae variables.