1 Introduction

$\sigma$ Gem (=HR 2973 = HD 62044) is a well known member of the long-period RS CVn systems. It is a non-eclipsing single-line spectroscopic binary with a long orbital period of 19.605 days (Bopp & Dempsey 1989). The visible primary is a red giant of spectral type K1 III. Little is known about the secondary component.

The light variability of this star was discovered by Hall et al. (1977). Fried et al. (1983) contributed a detailed photometric study of $\sigma$ Gem and derived a reliable photometric period of 19.423 days, which is shorter than the orbital period. The amplitude of the photometric wave is variable with a maximum value of 0.17 mag in V. As a hyperactive star $\sigma$ Gem draws much attention of astronomers. Focusing on the starspot changes over an eight-year period, Strassmeier et al. (1988) analyzed the V light curves and discussed the cyclic variations and spot migration. With a simplified light curve modeling method they suggested two cool spot regions on the K1 III primary and found a 2.7 yr period of cyclic variation for both spots and a new photometric period of 19.410 days. Later, Olah et al. (1989) investigated the light curve behavior of $\sigma$ Gem from 1986 to 1988 with the same method. They also confirmed large spot changes from year to year and the existence of two active longitudes on $\sigma$ Gem. A detailed study on spot activities of $\sigma$ Gem was recently published by Henry et al. (1995).

Strong CaII H and K emission, UV line emission and X-ray emission have also been observed, revealing the unusual manifestations of $\sigma$ Gem with high chromospheric and coronal activity (Schrijver et al. 1995; Elgaray et al. 1997). The H$_{\alpha}$ core emission has been observed by many authors. First, Smith & Bopp (1982) noted that the H$_{\alpha}$absorption line in $\sigma$ Gem was partly filled in by chromospheric emission. Strassmeier et al. (1986) measured the H$_{\alpha}$ feature with an average EW of 972 mÅ and an average $R_{\rm c}$ of 0.51. Strassmeier & Fekel (1990) and Frasca & Catalano (1994) computed the residual emission separately and got different results. A detailed study of the H$_{\alpha}$ profile by Eker (1986) showed the line core to vary with the orbital phase in agreement with the location of stellar active regions. Bopp et al. (1988) indicated that the H$_{\alpha}$ EW varied with rotational phase in 1986 but not obviously in 1987.

From 1993 to 1994, we made almost simultaneous photoelectric UBV and spectroscopic observations. In this paper, we present an analysis of the observations. The spot activity and the chromospheric H$_{\alpha}$ emission will be discussed in the context of the stellar evolutionary status.