1 Introduction

HR 7428 (=V 1817 Cygni) is a spectroscopic binary composed of a K2 II-III star (Ginestet et al. 1999) and a main sequence A2 star (Parsons & Ake 1987). Its long orbital period ($\simeq$108.6 days) has probably prevented systematic spectroscopic observations in the last decades. So, before the present work, the only available spectroscopic observations and orbital solution, for the primary cool component, have been done by Sanford (1925). Lucy & Sweeney (1971) revised that solution and concluded that the eccentricity is not significantly different from zero.

Ca  II  H & K emission was first reported by Gratton (1950), while Bopp & Talcott (1978) observed the H$\alpha$ line in absorption. Xue-fu & Hui-song (1986a) detected emission in both H$\alpha$ wings, but two later observations (Xue-fu & Hui-song 1986b) showed only variations in the H$\alpha$ profile without any emission. A single spectrum presented by Eker et al. (1995) showed a filled-in H$\alpha$ line. This observed behaviour indicates a large variability, probably due to chromospheric activity.

Barksdale et al. (1985) showed the system to be photometrically variable, the dominant cause being ellipticity, with a possible small reflection effect. Strassmeier et al. (1989) excluded reflection and estimated a starspot wave amplitude of 0.02 magnitudes from unequal heights of maxima and depths of minima in the light curve. A determination of physical parameters was performed by Hall (1990) combining the amplitude of the ellipticity effect, the mass function and the $v\sin i=21~{\rm km}\,{\rm s}^{-1}$ reported by Hui-song & Xue-fu (1987). A more recent and precise measurement of the projected rotational velocity, $v\sin i\,=\,$17.2$\,\pm\,$1.0 km$\,$s^-1, has been provided by De Medeiros & Mayor (1995).

Photometric observations between 1982 and 1989 were used by Hall et al. (1990a) for a detailed analysis of the photometric behaviour. After removing the modulation produced by the ellipticity effect, they were able to detect starspot signatures on the K-type primary star, as well as migration of spots. Gurzadyan (1997) included HR 7428 in his sample of binary stars for which he estimated the radii of the main components in the hypothesis of common chromospheres around binary systems.

In this work we present the results of our optical spectroscopy and determine new orbital and physical parameters. We estimate new temperatures and radii of both components comparing UBV magnitudes and low resolution IUE spectra with synthetic photospheric spectra. The Hipparcos photometry is also discussed, to constrain the ellipticity parameters.