Detection of a degenerate companion of the sx phoenicis star kz hydrae by studying its long-term variability

We present 109 new times of maximum light of the SX Phoenicis (SX Phe) star KZ Hydrae (KZ Hya) based mainly on our extensive photometric observations for two decades, leading to determination of a general ephemeris combined with the data in the literature. The O - C diagram reveals a continuously increasing period change combined with the light traveling time effect of the orbital motion of KZ Hya in a binary system with an orbital period of 26.8 0.2 years. The deduced mass of the companion cannot be smaller than 0.83 M ⊙. Since no sign of the companion has ever been observed spectroscopically and the observed color index b - y excludes the possibility of the companion being a late-type main-sequence or dwarf star, KZ Hya becomes the first SX Phe star for which a degenerate companion is detected. The pulsation properties are studied by analyzing the Fourier spectra of the observed light curves while the fundamental parameters are deduced with the simultaneous multicolor uvbyβ data, showing this star to be a strong low-metallicity high-amplitude SX Phe-type variable pulsating in the fundamental radial mode. No signs of multiperiodicity or significant long-term changes in amplitude are detected in the pulsation of this variable. © 2008. The American Astronomical Society. All rights reserved.