2 Observations

The observations were carried out on different nights of September-October 1996 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, 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. C1=SAO 92273 ( $V=8\hbox{$.\!\!^{\rm m}$ }5$, K0) was used as the main comparison star and C2=SAO 92119 ( $V=10\hbox{$.\!\!^{\rm m}$ }0$, G0), C3=SAO 92152 ( $V=9\hbox{$.\!\!^{\rm m}$ }4$, G5) and C4=SAO 74083 ( $V=6\hbox{$.\!\!^{\rm m}$ }7$, A2) as check stars. 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 the main comparison star (C1=SAO 92273), 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 standard values of C4, $V=6\hbox{$.\!\!^{\rm m}$ }70$, $b-y=0\hbox{$.\!\!^{\rm m}$ }046$, $m_1=0\hbox{$.\!\!^{\rm m}$ }161$ and $c_1=1\hbox{$.\!\!^{\rm m}$ }193$, listed in the Hauck & Mermilliod (1998) catalogue. In addition, we obtained the following values of $V=8\hbox{$.\!\!^{\rm m}$ }52$, $10\hbox{$.\!\!^{\rm m}$ }08$, $9\hbox{$.\!\!^{\rm m}$ }33$ for C1, C2 and C3, respectively.