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We used a ~300 ks long XMM-Newton observation of the Seyfert 1 galaxyMCG-6-30-15 to study the correlation between the 0.2-10 keV X-ray and the3000-4000 A bands. We found a significant correlation peak at a time lag of 160ks where the UV flux variations preceded the variations in the X-ray band. Weinterpret this result as evidence in favour of Comptonisation models where theobserved X-rays are produced through Compton up-scattering of thermal UV seedphotons from an accretion disc, as this process naturally predicts the UVvariations to precede similar flux variations in the X-rays. The length of thetime lag favours models where the observed UV and the seed-photon-emittingregions are connected by perturbations of the accretion flow traveling inwardsthrough the disc, affecting first the main U-band-emitting radii and then theinnermost region where the bulk of the seed photons is expected to be produced.Finally, the absence of significant features in the correlation function withX-ray flux variations preceding those in the UV indicates that the observedU-band photons are not mainly produced through reprocessing of hard X-rays inthis source.Comment: 9 pages, 8 figures, accepted for publication in A&

X-ray to UV variability correlation in MCG–6-30-15