1 Introduction

Study of rapid intensity variations of active galactic nuclei provides a uniquely powerful tool for investigating the processes occurring in the vicinity of their central engines. This strategy is further sharpened when the variability is monitored simultaneously in different frequency bands. The flat-spectrum radio source S5 0716+71, which is an intra-day variable at radio and optical wavelengths as well as a $\gamma-$ray emitter (Wagner et al. 1996) is a favourite target for variability studies. This BL Lac object (Biermann et al. 1981) is located at a redshift $z~\ge~0.3$ (Schalinski et al. 1992) and its radio structure shows a double structure reminiscent of the FR II morphology (Antonucci et al. 1986). Remarkably, in a 4-week long monitoring campaign in February 1990, this blazar showed an abrupt transition in its quasi-periodic variability pattern from $\sim$1-day timescale during the first week, to $\sim$6-day timescale subsequently, both at optical and centimetre wavelengths (Quirrenbach et al. 1991; Wagner 1991). This strong correlation between the optical and centimetre regimes provided a strong hint that the observed rapid radio variability is not due to propagation effects in the intervening media. Another interesting result is that the radio spectral index of this blazar was found to correlate with intra-night optical variations, such that the radio spectrum flattens near the optical maxima for several cycles (Qian et al. 1991; Wagner et al. 1996). Tendency for the optical spectrum (B-I) to harden during the bright optical phase has been reported in Wagner et al. (1996). These authors further inferred that in the R-band light curves a significant signature of "flickering'' on time scale as short as $\approx$15 min, i.e., much faster than the observed quasi-periodicity on $\sim$1 day time scale.

Recently, Ghisellini et al. (1997) have reported results of monitoring of this blazar in the B,V,R,I colours during November 1994 to April 1995. They have also provided in these passbands the calibrated magnitudes of 4 comparison stars in the field of this blazar for which they have estimated an interstellar extinction of A_V=0.23 mag. Their campaign recorded a few events of ultra-rapid (intra-night) variations with an amplitude of about $5\%$ within a few hours, superposed on slower variations. Another finding from their study is that in the "low'' optical state the (B-R) spectral index responds to fast optical variations occurring on day-like time scales, the spectrum becoming bluer with increasing brightness in R. Interestingly, such a correlation was not found for the slower variations, indicating a different physical mechanism for such variations (Ghisellini et al. 1997).

In order to investigate further this blazar, a one-month long multi-band monitoring campaign covering radio through $\gamma-$ray bands was carried out by us and our collaborators during February/March 1994. In the optical, only R band was common to all the datasets obtained from the various participating observatories. A detailed presentation based on this campaign is under preparation. In this brief paper, we confine ourselves to presenting the results from our dense multi-colour optical monitoring. These observations covered a total baseline of one month and provided a fairly dense coverage on most of the nights. Such data can allow us to resolve the temporal structure of the intensity variations, thereby providing a closer insight into the radiation processes involved.