2 Observations

The observations were carried out on different nights of September and October 1997 using the 90 cm telescope at Sierra Nevada Observatory, Spain. This telescope is equipped with a six channel uvby$\beta$ photometer for simultaneous measurements in uvby or in the H$_{\beta}$ channels, respectively (Nielsen [1983]), but only uvby measurements were collected during this observing run.

In order to make differential photometry, different sets of comparison stars were used during September and October. The comparison stars were chosen taking into account their spectral type (close to solar) and in the neighbourhood of the asteroids for better reduction of the data avoiding extinction problems. Each set of comparison stars contains one main comparison and one check star. During September observations C1=SAO 146842 ( $V=7\hbox{$.\!\!^{\rm m}$ }2$, F8) was used as the main comparison star and C2=SAO 146908 ( $V=7\hbox{$.\!\!^{\rm m}$ }6$, F8) as a check star. During October observations C3=SAO 165708 ( $V=6\hbox{$.\!\!^{\rm m}$ }4$, G0) was used as the main comparison star and C4=SAO 165638 ( $V=7\hbox{$.\!\!^{\rm m}$ }7$, G5) as a check star. In order to make the two sets of comparison stars compatible the main and the check stars of both sets were observed simultaneously during different nights.

The observing and reduction procedure was standard, based on consecutive measurements of the selected main comparison star, check comparison star, asteroids and sky reading. During the observations reported here, neither of the comparison stars showed any sign of variability within $0\hbox{$.\!\!^{\rm m}$ }005$.

To transform our data into the standard uvby system we have used the same procedure described in Rodríguez et al. ([1997]). After differential magnitudes in the standard system were obtained for the asteroids with respect to one main comparison star (C1=SAO 146842) we correct them to unit distance from the sun and the earth and perform light-time corrections to all the observations. Then, we transform these differential magnitudes to absolute magnitudes using the absolute values of C1, $V=7\hbox{$.\!\!^{\rm m}$ }156$, $b-y=0\hbox{$.\!\!^{\rm m}$ }310$, $m_1=0\hbox{$.\!\!^{\rm m}$ }163$ and $c_1=0\hbox{$.\!\!^{\rm m}$ }413$, listed in the Hauck & Mermilliod ([1998]) catalogue.

In addition, we obtained the following values of $V=7\hbox{$.\!\!^{\rm m}$ }656$, $6\hbox{$.\!\!^{\rm m}$ }377$, $7\hbox{$.\!\!^{\rm m}$ }750$, $b-y=0\hbox{$.\!\!^{\rm m}$ }290$, $0\hbox{$.\!\!^{\rm m}$ }548$, $0\hbox{$.\!\!^{\rm m}$ }589$, $m_1=0\hbox{$.\!\!^{\rm m}$ }165$, $0\hbox{$.\!\!^{\rm m}$ }269$, $0\hbox{$.\!\!^{\rm m}$ }345$, and $c_1=0\hbox{$.\!\!^{\rm m}$ }517$, $0\hbox{$.\!\!^{\rm m}$ }427$, $0\hbox{$.\!\!^{\rm m}$ }357$ for C2, C3 and C4, respectively, in good agreement with the values listed in Hauck & Mermilliod ([1998]).