3 The new light curves

The new UBV light curves (Fig. 1) are single-humped with asymmetry. The maximum brightness located around $\phi = 0.2$ is found to be about 4.18 mag in V, 5.32 mag in B and 6.28 mag in U, respectively. The minimum is located around $\phi =0.6$. The amplitudes of the light variation of $\sigma$ Gem in 1993/94 was about 0.1 mag, 0.09 mag and 0.07 mag in V, B and U respectively. Comparing with the light curves of Olah et al. (1989) obtained in 1986, 1987 and 1988, we find that the distortion wave has evidently been migrating steadily.

The shape of the light curves implies large-scale spot activity on surface of the star. To study the spot activity furtherly from the light curves, a brief photometric analysis was carried out using the Wilson-Devinney program with spot approximation (Kang & Wilson 1989; Zhai et al. 1994). The major Roche parameters are mainly adopted or estimated from the previous works on this system (Stawikowski & Glebocki 1994; Eker 1986). With a two-spot model, the computed light curves give a good fitting to all the V, B and U light curves as shown in Fig. 1.

The result indicates that probably two cool spots exist on the primary component of $\sigma$ Gem in the season of 1993/94. The one cooler and larger is located at the phase around $\phi =0.6$ and the other, smaller one is around the phase of $\phi=0.9$.Additional spots should also be present on the star to account for the general dimming from the unspotted level (4$.\!\!^{\rm m}$137, Strassmeier et al. 1988) to our measured maximum light (4$.\!\!^{\rm m}$18). Because of the considerable observational errors and the large gap between our two observational intervals, however, the resulting parameters of the spots are uncertain.