1 Introduction

YY Eri (BD $-10^{\circ}858$ = SAO 149449, m_v=8.05-8.80, $p=0\hbox{$.\!\!^{\rm d}$}3125$, sp. type G5V) was classified as a W UMa-type binary by Bodokia (1938). The first photoelectric observations were made by Cillie (1951); then it was observed again by Huruhata et al. (1953), Purgathofer et al. (1960), Budding (1983), Eaton (1985), Müyesseroglu et al. (1990) and Maceroni et al. (1994). Light curve solutions were presented by Huruhata et al. (1953), Binnendijk (1965), Maceroni et al. (1982), Eaton (1986), Nesci et al. (1986), Maceroni et al. (1994) and Budding et al. (1997). The studies of the orbital period change were conducted by Cillie (1951), Huruhata et al. (1953), Kwee (1958), Bhattacharyya (1967), Kim (1992) and Maceroni & Van't Veer (1994). The spectrographic observations performed by Struve (1947) were re-analyzed by Huruhata et al. (1953). They determined a spectroscopic mass ratio of 0.59, which is discrepant with the photometric mass ratio. This question was resolved by the new radial velocity study (Nesci et al. 1986).

The surface activity of the system was revealed by observations in various bands. Light curve asymmetry perturbations shifting in phase as evidence of photospheric spots (Maceroni et al. 1994) were exhibited by photometric observations. The emission lines as signatures of chromospheric activity were found in IUE spectra (Eaton 1983). And the system also appears to be an X-ray source in the survey of the Einstein satellite (Cruddace & Dupree 1984).