1 Introduction

TZ Eridani (BD $-6\hbox{$^\circ$}$880) is an Algol-type EA eclipsing binary of short period, P = 2.606 days. The total primary eclipse is 2.5 mag deep in the V band. The estimation of the spectral type of the two components is F (Cannon 1934), F8 (Brancewicz & Dvorak 1980) for the primary, and K0IV (Kaitchuck & Park 1988), K0 (Yoon et al. 1994) for the secondary. However, as shown in this paper, the most probable spectral types are A5/6 V for the primary and K0/1 III for the secondary.

A first estimate of the physical parameters of the components is given by Brancewicz & Dvorak (1980), in their "Catalogue of Parameters for Eclipsing Binaries'', in particular R₁= 1.51 $R_{\odot}$ (primary component) and R₂= 1.92 $R_{\odot}$. Recall that these values have not been obtained by a simultaneous treatment of the complete light and radial velocity curves. It is the aim of this paper to perform such a new analysis, by using the method and the computer programme of Wilson & Devinney (1971).

No complete light or radial velocity curves have been yet published for TZ Eri. For that reason, this star was measured intensively in the 7-colour Geneva photometric system (Golay 1980; Rufener 1988) using the Swiss telescope at La Silla (European Southern Observatory, Chile) equipped with the two-channel aperture photometer P7 (Burnet & Rufener 1979). Moreover, the radial velocity curve of the primary component has been determined with the spectrovelocimeter CORAVEL installed on the 1.54 m Danish telescope at La Silla, and with the Illinois Cassegrain ("white'') spectrograph attached to the 1 m Illinois reflector at Mount Laguna Observatory. In addition, one spectrum of the secondary component has been obtained by using the NTT 3.5 m telescope at La Silla, allowing the determination of the masses of the two components of TZ Eri.

TZ Eri was discovered to have a disk (Kaitchuck & Honeycutt 1982) on the basis of emission in H$_{\beta}$ and H$_{\gamma}$ lines observed during the eclipse. Kaitchuck & Park (1988) showed that this disk belongs to the class of the transient accretion disks which are produced by a collision of the gas stream with the mass-gaining star (primary component). The radius of the disk was measured by the duration of the presence of H$_{\beta}$ line in emission after the start of the primary eclipse (second contact) or before the end of the same eclipse (third contact). The disk extension is variable from one eclipse to another, and in most cases between the trailing and leading sides. During the 12 eclipses studied, the disk extension r/R₁ varied from 1.0 to 1.64.

In this paper, the variation of the period of TZ Eri will be analysed, the variability of the components will be examined, and the physical parameters of the two components will be determined from the analysis of the light and radial velocity curves.