Discovery of the spectroscopic binary nature of the Cepheids X Puppis and XX Sagittarii

TL;DR

This study identifies X Puppis and XX Sagittarii as spectroscopic binary Cepheids based on 75 years of data, highlighting the high binary occurrence among classical Cepheids and its implications for period-luminosity calibration.

Contribution

It provides the first spectroscopic binary classification for these Cepheids, emphasizing the importance of binary systems in Cepheid studies.

Findings

Both Cepheids are confirmed as spectroscopic binaries.

X Puppis shows a steadily increasing pulsation period.

XX Sagittarii's pulsation period remains stable over a century.

Abstract

We present the analysis of photometric and spectroscopic data of two bright Galactic Cepheids, X Puppis and XX Sagittarii. Based on the available data in the literature as well as own observations spanning 75 years, we conclude that both Cepheids belong to spectroscopic binary systems. However, the data are not sufficient to determine the orbital periods nor other elements for the orbit. This discovery corroborates the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. The photometric data revealed that the pulsation period of X Pup is continuously increasing with Delta P=0.007559 d/century likely caused by stellar evolution. The pulsation period of XX Sgr turned out to be very stable in the last ~100 years.