1 Introduction

The light variability of YY CMi ($\equiv$HD67100) was discovered by Morgenroth (1934). Later photometric observations were due to Lause (1938), Soloviev (1940), Kaho (1950), and Kordylewsky & Szafraniec (1957). According to the General Catalogue of Variable Stars (GCVS, III ed., Kukarkin et al. 1969), the system is classified as a $\beta$ Lyrae type eclipsing binary with a period of 1.0940253 d and a spectral type F5. Later, the spectrum was classed by Hill et al. (1975) as F6V at phase 0.31 and F7V at phase 0.71. The first complete light curve in three colours (u, b, y) was obtained by Abhyankar (1962), who also presented a solution based on Russell and Merrill method. From a questionable treatment of the colour indices, Abhyankar (1962) concluded that the system is composed of an F6III primary and an A5V secondary. Koch et al. (1970) noticed that YY CMi was probably a system of two (F5 + F8) main sequence stars.

Giuricin & Mardirossian (1981) reanalyzed Abhyankar's (1962) three-colour photoelectric observations by using Wood's (1972) model and found a solution appreciably different from the previous ones. The elements they derived lead to an evolved contact system consisting of a primary (roughly an F6 star) and a secondary (early G5) of practically equal sizes. This picture of the system is only an approximation of the real one, since Wood's model treats the stars as triaxial ellipsoids and does not handle contact systems very well.