CY Aquarii: A Triple System with Twin Companions

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-year observational dataset, augmented by 1,367 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 $(1/P_0)(\mathrm{d} P/ \mathrm{d} t) = (2.132 \pm 0.002) \times 10^{-8} \mathrm{yr}^{-1}$ 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 years and Companion B has an orbital period of approximately 50.8 years. It is highly probable that both Companion A and B are white dwarfs, with Companion A's elliptical orbit displaying an eccentricity of $e = 0.139 \pm 0.002$, 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.