2 Observations

2.1 Spectroscopy

All spectroscopic observations were obtained with the McMath-Pierce telescope at Kitt Peak National Solar Observatory (NSO). Our 52 spectra were taken during a 70 nights long observing run between 1 November 1996-9 January 1997, covering 3.57 consecutive stellar rotations. We used the stellar spectrograph with the $800\times 800$ TI-4 CCD camera at a dispersion of 0.10 Å/pixel and a resolving power of 42000 as judged from the width of several Th-Ar comparison-lamp lines. The observations covered the 6410-6460 Å wavelength range but include only two lines suitable for Doppler imaging (Ca I 6439 and Fe I 6430) due to a cosmetic defect on the detector. The average signal-to-noise (S/N) ratio is $\approx$400:1 but many spectra have more than 500:1 and several also near 300:1.

Data reductions were done using the NOAO/IRAF software package and followed our standard procedure for coudé spectra (see, e.g., Weber & Strassmeier 1998). Nightly observations of the radial-velocity standard $\alpha$ Ari were used to obtain the radial velocities for $\sigma$ Gem, except for one night where $\beta$ Gem was used instead. We adopted $v_{\alpha {\rm Ari}}=-14.51$ kms^-1 and $v_{\beta {\rm Gem}}=+3.23$ kms^-1 (Scarfe et al. 1990). Table 1 gives the observing log and the radial velocities.

Our data are phased with the orbital period from Dümmler et al. (1997) and from a recent time of maximum positive velocity according to our Table 1

$${\rm HJD}=2\,450\,388.36853 + 19.60447 \times \ E.$$ (1)

2.2 Photometry

Photometric data were collected between 8 November 1996-21 January 1997 with the Wolfgang 0.75-m automatic photoelectric telescope (APT) of the University of Vienna and located at Fairborn Observatory in Arizona (Strassmeier et al. 1997). The telescope was equipped with Strömgren b and y filters. Altogether, 46 new by measurements were obtained during the spectroscopic campaign, each of them being the mean of eight readings per night on average. HD60318 was used as the comparison star (Hipparcos magnitudes of HD60318 are $V=5\thinspace\hbox{$.\!\!^{\rm m}$ }34$, $B-V=1\thinspace\hbox{$.\!\!^{\rm m}$ }010$, $V-I=0\thinspace\hbox{$.\!\!^{\rm m}$ }99$; ESA 1997).