1 Introduction

ON 231 (1219+285, z=0.102) is one of the few radio sources whose optical counterpart was already known as a variable star (W Com) discovered by Wolf (1916), at the epoch of its classification as a BL Lacertae object (Browne 1971).

Its historic light curve dates back to more than 100 years and shows several interesting features (see Sect. 4). Most of the observations of ON 231 reported during the past 30 years were obtained photographically mainly in the B band and only a few data have been obtained via photoelectric or CCD photometry in the BVRI bands.

Furthermore, after the photographic B band observations taken during the Rosemary Hill monitoring program reported by Webb et al. (1988), only sporadic data are available (e.g., Xie et al. 1990, 1992).

Massaro et al. (1992) found the source at the highest brightness level since 1968. This high state was qualitatively confirmed by the work of Schramm et al. (1994): they reported an uncalibrated R (Johnson) light curve of ON 231 obtained during the period 1989-1992, from which an increasing trend of the source brightness is evident over all their observational period, with a maximum in 1992. More recent observations of ON 231 are lacking in the literature.

It is known that the source shows variability on time scales from hours to years (see, e.g., Smith & Nair 1995; Xie et al. 1992). But previously published light curves are not so densely sampled to allow a sufficiently detailed study of structure and occurrence of the short term variations.

Worral et al. (1986) and Lorenzetti et al. (1990) found that the optical-near-infrared spectrum of ON 231 becomes steeper when the source is fainter. In the optical band this effect has been attributed to the underlying galaxy (Worral et al. 1986), but Lorenzetti et al. (1990) excluded the host galaxy as the possible cause of the near-infrared spectral variability observed in ON 231. Also in this case the spectral analysis of the optical continuum was performed only over a limited period of time and during relatively faint states of the source: there is no information on the behaviour of the spectral slope during a large outburst or during rapid flares.

Moreover, $\gamma$-ray emission from ON 231 has been detected by the EGRET experiment on board the Compton Gamma Ray Observatory CGRO (von Montigny et al. 1995). This $\gamma$-ray emission was revealed only above 300 MeV. The derived photon spectral index was $\sim~1.27$, which makes the ON 231 spectrum one of the hardest ever observed within the whole EGRET blazars sample (Sreekumar et al. 1996). The source was included in the supplement to the second EGRET Catalog, built after the completion of the CGRO phase 3 (Thompson et al. 1996).

According to the current models, the optical emission could be strongly coupled to the high energy $\gamma$-rays (see, e.g., Ghisellini et al. 1997). In this contest, simultaneous observations in the optical and $\gamma$-ray bands could be crucial to understand the nature of the $\gamma$-ray emission in blazars.

Starting from the above considerations, ON 231 was intensively observed within a coordinated optical monitoring of a selected sample of $\gamma$-ray loud BL Lac objects carried out by the Perugia, Roma, Torino and Tuorla groups.

In this paper we present and discuss in detail the $BVR_{\rm c}I_{\rm c}$ light curves of ON 231 obtained during the period 1994-1997, when the source was found at the highest brightness state ever observed.