2 Observations and data reductions

In the 1993/94 observing season, new UBV photometry of $\sigma$ Gem was carried out at the Xinglong station of Beijing Astronomical Observatory. On 17 nights from 1993 January to 1994 January, the observations were made on the 60 cm reflector using a single channel photometer with a direct amplification receiver. The star $\rho$ Gem (=HD 58946, V=4.18, U-B=-0.03, B-V=+0.32, Strassmeier et al. 1988) was adopted as the comparison star. The integration time was 30 s for both the target and comparison stars in all filters. The accuarcy of the observations was estimated to be about 0.01 mag in all V, B and U bands on most nights. All the measurements were transformed to the UBV standard system and combined into light curves using the photometric ephemeris of Fried et al. (1983),

$${\rm JD(hel)} = 2444686\hbox{$.\!\!^{\rm d}$}5 + 19\hbox{$.\!\!^{\rm d}$}423 \times E.$$

Table 1 gives the dataset of UBV measurements. We combined the data obtained on one night into one average point because of the long period of the system. The UBV light curves are shown in Fig. 1, where circles and triangles represent the measurements observed in early 1993, and from 1993 November to 1994 January.

 

Table 1: Photometric measurements of $\sigma$ Gem

  

Figure 1: The new light curves of $\sigma$ Gem observed in 1993/94. Circles denote the measurements obtained in 1993 Jan.-Feb., and the triangles stand for that observed in 1993 Nov.-1994 Jan. Solid lines represent the computed light curves using W-D method with two-spot model

  

Figure 2: Average spectrum of $\sigma$ Gem of the H$_{\alpha}$ order after velocity shift corrections. The dotted curves illustrate the Gaussian fittings to the H$_{\alpha}$ profiles

Spectroscopic observations were made with the 2.16 m telescope also at the Xinglong station in 1993 on eight nights from 1993 Jan. 12 to Jan. 19. The spectra were obtained by using the fiber-fed Cassegrain ISIS spectrograph, which was designed specifically for the international MUSICOS observations by Meudon Observatory (Cuby et al. 1992). With a THX 576$\times$384 CCD detector the ISIS spectrograph yields a resolving power of about $3.6\ 10^{4}$ at H$_{\alpha}$ band. The spectrum covers about 35 Å  from 6544.5 to 6579.5 Å. The reciprocal dispersion is 2.64 Å mm^-1, corresponding to a 2-pixel resolution of 0.122 Å. The star $\beta$ Gem (K0 III) was used as comparison star.

On most nights, we obtained two spectra of the target, one before midnight and one after midnight. A total of 14 spectra with high S/N ranging from 160 to 300 were obtained. The spectra obtained on Jan. 16 are noisy because of the bad weather. Information about the observations is given in Table 2, where JD of the observations corresponds to the midexposure and the phases were computed from the ephemeris of Fried et al. (1983).) Each spectrum was reduced according to the usual procedures: bias subtraction, dark subtraction, flat-field division, global slope correction and wavelength calibration. The precision of the calibration is better than 0.02 Å. The continuum level is established by choosing two line-free windows, approximately 2 Å wide, centered at 6550 and 6578 Å. A straight line is then drawn through these two regions to define the continuum in each spectrum. Two representative spectra illustrating our continuum definition are shown in Fig. 2. The upper one is the average spectrum of $\sigma$ Gem and the lower one is of the comparison, $\beta$ Gem.

 

Table 2: The spectroscopic observations of $\sigma$ Gem in 1993