Do Miras show long secondary periods?

Abstract

Aims. The long secondary period (LSP) phenomenon, which is commonly observed in pulsating red giants, has not been detected in any Mira yet. The goal of this work is to verify, if there is a physical reason for this or if it is simply an observational bias. Methods. The OGLE-III Sample of Long Period Variables in the Large Magellanic Cloud, containing 1663 Miras, is used to perform a search for secondary periodicity in these objects and identify candidates for the long secondary period stars based on the location on the period-luminosity diagram. Results. Out of 1663 Miras, 108 were identified as potential candidates, with variability broadly consistent with LSP. This makes 7% of the whole Mira sample in the Large Magellanic Cloud. Most, if not all, of the Mira LSP candidates are C-rich stars. Conclusions. The results of this analysis suggest that Miras may exhibit long secondary periods. However, the long-term variability can also be related to the period and amplitude irregularities that Miras are known to exhibit. Further study will be necessary to draw a definitive conclusion.