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In this paper we report on the analysis of all the available optical and veryhigh-energy $\gamma$-ray ($>$200 GeV) data for the BL Lac object PKS2155$-$304, collected simultaneously with the ATOM and H.E.S.S. telescopes from2007 until 2009. This study also includes X-ray (RXTE, Swift) and high-energy$\gamma$-ray (Fermi-LAT) data. During the period analysed, the source wastransitioning from its flaring to quiescent optical states,and wascharacterized by only moderate flux changes at different wavelengths on thetimescales of days and months. A flattening of the optical continuum with anincreasing optical flux can be noted in the collected dataset, but onlyoccasionally and only at higher flux levels. We did not find any universalrelation between the very high-energy $\gamma$-ray and optical flux changes onthe timescales from days and weeks up to several years. On the other hand, wenoted that at higher flux levels the source can follow two distinct tracks inthe optical flux-colour diagrams, which seem to be related to distinct$\gamma$-ray states of the blazar. The obtained results therefore indicate acomplex scaling between the optical and $\gamma$-ray emission of PKS2155$-$304, with different correlation patterns holding at different epochs,and a $\gamma$-ray flux depending on the combination of an optical flux andcolour rather than a flux alone.

Long-term monitoring of PKS 2155$-$304 with ATOM and H.E.S.S.: investigation of optical/$\gamma$-ray correlations in different spectral states