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We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bandsin 2002. The observations resulted in almost evenly sampled light curves, 3-8hours long, with an average sampling interval of ~5-15 min. Because of thedense sampling and the availability of light curves in more than one opticalbands we are able to study the intra-night flux and spectral variability of thesource in detail. Significant observations were observed in all but one cases.On average, the flux variability amplitude, on time scales of minutes/hours,increases from ~2-3% in the I, to ~3.5-5.5% in the B band light curves. We donot detect any flares within the individual light curves. However, there is apossibility that the April 2001 and late May 2002 observations sample twoflares which lasted longer than ~1-3 days. The evidence is only suggestivethough, due to the limited length of the present light curves with regard tothe duration of the assumed flares. No spectral variations are detected duringthe April 2001 observations. The source flux rises and decays with the samerate, in all bands. This variability behaviour is typical of S4 0954+658, andis attributed to geometrical effects. However, significant spectral variationsare observed in May 2002. We find that the spectrum hardens/softens as the fluxincreases/decreases, respectively. Furthermore, the "hardening" rate of theenergy spectrum is faster than the rate with which the spectrum becomes"redder" as the flux decays. We also find evidence (although of low statisticalsignificance) that the I band variations are delayed with respect to the B bandvariations. If the May 2002 observations sample a flaring event, these resultssuggest that the variations are caused by energetic processes which areassociated with the particle cooling and the source light travel time scales.Comment: 7 pages, 7 figures, accepted for publication in A&

Multi-band optical micro-variability observations of the BL Lac object S4 0954+658