CY Aquarii: A Triple System with Twin Companions

Abstract In this study, we conducted a comprehensive analysis of the SX Phoenicis (SX Phe) type star CY Aquarii (CY Aqr). Our investigation included a detailed O − C analysis based on a 90 yr observational data set, augmented by 1367 newly determined times of maximum light. The O − C diagram reveals that (i) the primary star of CY Aqr exhibits a linear period variation rate of (1/ P 0 )( d P / dt ) = (2.132 ± 0.002) × 10 −8 yr −1 for its dominant pulsation mode; (ii) the primary star is disturbed by two companions and part of a triple system; (iii) Companion A has an orbital period of approximately 60.2 yr and Companion B has an orbital period of approximately 50.8 yr. It is highly probable that both Companion A and B are white dwarfs, with Companion A’s elliptical orbit displaying an eccentricity of e = 0.139 ± 0.002, which is the lowest confirmed value in similar binary and triple systems to date. Most notably, Companion A and B have masses that are identical within the uncertainties, with a mass ratio exceeding 0.99. Whether this is considered a coincidental event or the result of an underlying mechanism, CY Aqr is an exceptionally rare case that broadens our understanding of multiple star systems and offers a unique opportunity to delve into the enigmatic evolutionary histories of such configurations. Further intriguing characteristics of this system warrant investigation in future studies, based on additional observational data.