We present spectroscopic time series observations on one of the brightest northern RRc star, T Sex. Additionally, we also analysed extended photometric data sets, particularly the recent observations of the TESS space telescope. The main findings of our studies are as follows: T Sex, unlike all RRc stars whose space photometry has been analysed, shows only the $0.5f_x$ frequency as an additional pulsation frequency. With this, T Sex may be the first represent of such rare RRc stars found from space photometry. The spectroscopic data show a periodic distortion of the H$\alpha$ line with the pulsation phase. This phenomenon has not been reported for any RR Lyrae stars. The characteristic line distortion is probably caused by the turbulent convection, which resulted in higher macroturbulent velocity for T Sex than for typical RRab stars. Line doubling of the Na D line was observed between the 0.37 and 0.80 pulsation phases. The explanation of this phenomenon is that the two absorption components originate from different sources. The redder component comes from the pulsating atmosphere of the star, while the bluer one from the interstellar space. At phase 0.438, we detected emission on the Na D line, which may indicate a weak shock wave.

中文翻译：

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明亮的 RRc 星 T Sex 的光谱和光度时间序列

我们展示了对最亮的北极 RRc 星之一 T Sex 的光谱时间序列观测。此外，我们还分析了扩展的光度数据集，特别是最近对 TESS 太空望远镜的观测。我们研究的主要发现如下：T Sex 不像所有 RRc 恒星的空间光度测量已被分析，仅显示 $0.5f_x$ 频率作为额外的脉动频率。有了这个，T Sex 可能是从空间光度测量中发现的这种罕见的 RRc 恒星的第一个代表。光谱数据显示了 H$\alpha$ 线随脉动相位的周期性失真。对于任何天琴座 RR 星都没有报道过这种现象。特征线失真可能是由湍流对流引起的，这导致 T Sex 的宏观湍流速度高于典型的 RRab 恒星。在 0.37 和 0.80 脉动阶段之间观察到 Na D 线的线加倍。对这一现象的解释是两种吸收成分的来源不同。较红的成分来自恒星的脉动大气，而较蓝的成分来自星际空间。在相位 0.438，我们检测到 Na D 线上的发射，这可能表明存在微弱的冲击波。