1 Introduction

UX Ari is one of the most active RS CVn binaries that always show H$_\alpha$ emission above continuum. It is a non-eclipsing double-lined spectroscopic binary with spectral type G5V/K0IV (Carlos & Popper 1971) and orbital period 6$\fd$4378553 (Duemmler & Aarum 2001). The observations of several chromospheric activity indicators (Frasca & Catalano 1994; Huenemoerder et al. 1989; Montes et al. 1995; Montes et al. 1996; Montes et al. 2000; Nations & Ramsey 1986) have demonstrated that the chromospheric activity of UX Ari is very strong in the group of RS CVn binaries and the chromospheric activity of this system is associated with the cooler secondary component.

Recently, the spectral subtraction technique has been used widely for chromospherically active binaries in several lines Ca II H&K, Mg Ib, He I D₃, Na I D₁, D₂, H$_\alpha$, H$_\beta$, Ca II IRT (Gunn & Doyle 1997; Gunn et al. 1997; Lázaro & Arévalo 1997; Montes et al. 1997; Montes et al. 2000 etc.). By using this method, more and more information about chromospheric activity on stars has become known. This is very important for us to well understand the behaviour of chromospherically active binaries.

Here new spectroscopic observations of UX Ari are analyzed by using the spectral subtraction technique, and the excess emissions in several chromospheric activity indicators are determined. Based on these observations and analyses, some results are derived for the chromospheric activity of UX Ari. In Sect. 2, we give details about our observations and data reduction. In Sect. 3, we describe the procedure of spectral analysis. In Sect. 4, the behaviours of different chromospheric activity indicators are reported. Finally, the discussion and conclusions are given in Sect. 5.