4 Classification of the components and interstellar extinction

A photometric classification of the components of TZ Eri can be obtained because the primary eclipse is total, by using various calibrations based on the colour indices and reddening free parameters of the Geneva photometric system.

From the magnitudes in Table 2, the various Geneva colour indices and reddening free parameters d, $\Delta$ and g (see Golay 1980) have been calculated. Then, the technique of the photometric boxes (Golay et al. 1969; Nicolet 1981a) has been applied to determine the intrinsic characteristics of the primary component. Recall that this technique is specific to the Geneva photometric system and depends on the homogeneity and precision of its measurements. The assumption in this technique is that the properties of the stars lying within a specified photometric neighbourhood (see Nicolet 1994) can be equated with each other, provided that the radius of the photometric box is small enough. We applied the technique to the parameters d, $\Delta$ and g of the primary and found in the entire photometric database 40 twin stars, i.e. stars having very similar values of the 3 parameters, the radius of the photometric box having been chosen equal to 0.015 mag. It is assumed that these stars are intrinsically similar to the primary of TZ Eri. The same method was applied to the classification of the components of the eclipsing RS CVn-type system RZ Eridani (Burki et al. 1992).

It is especially noteworthy that 6 of these twin stars belong to open clusters: $\alpha$Per, Hyades, Praesepe, Pleiades (2 stars) and NGC 6405. This allowed to determine unanbiguously the intrinsic colours of these stars, since these clusters have well determined distances and interstellar extinctions. By using the values given by Nicolet (1981b), we derived the intrinsic colours of the twin stars, and thus also of the primary of TZ Eri, in particular $[B2-V1]_0 = 0.011 \pm 0.016$. According to the relations between spectral types and intrinsic Geneva colours or parameters by Hauck (1994) and to the calibration of Geneva photometry based on Kurucz's atmosphere models by Künzli et al. (1997), the estimated spectral type of the primary is A5/6 V, its effective temperature is $7770 \pm 100$  K, its gravity is $\log g = 4.40 \pm 0.07$ and its metallicity is $[M/H] = 0.13 \pm 0.09$. On the other hand, the colour excess is $E[B2-V1] = 0.047 \pm 0.017$.

For the secondary, we have calculated the intrinsic colours by using the measured values (see Table 1) and the colour excesses obtained for the primary. We obtained in particular [B2-V1]₀ = 0.756, an estimated spectral type of K0/1 III and an effective temperature of 4520 K.