2012MNRAS.426.3154S

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 our 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 or other elements for the orbit. This discovery corroborates statements on the high frequency of occurrence of binaries among 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 by ΔP = 0.007559 d/century, likely caused by stellar evolution. The pulsation period of XX Sgr turned out to be very stable over the last ∼ 100 years.