  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.
