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[ABRIDGED]We study the carbon abundances with a twofold objective. On the onehand, we want to evaluate the behaviour of carbon in the context of Galacticchemical evolution. On the other hand, we focus on the possible dependence ofcarbon abundances on the presence of planets and on the impact of variousfactors (such as different oxygen lines) on the determination of C/O elementalratios. We derived chemical abundances of carbon from two atomic lines for 757FGK stars in the HARPS-GTO sample. The abundances were derived with the codeMOOG using automatically measured EWs and a grid of Kurucz ATLAS9 atmospheres.Oxygen abundances, derived using different lines, were taken from previouspapers in this series and updated with the new stellar parameters. We find thatthick- and thin-disk stars are chemically disjunct for [C/Fe] across the fullmetallicity range that they have in common. Moreover, the population ofhigh-$\alpha$ metal-rich stars also presents higher and clearly separated[C/Fe] ratios than thin-disk stars up to [Fe/H]\,$\sim$\,0.2\,dex. The [C/O]ratios present a general flat trend as a function of [O/H] but this trendbecomes negative when considering stars of similar metallicity. We findtentative evidence that stars with low-mass planets at lower metallicities havehigher [C/Fe] ratios than stars without planets at the same metallicity, in thesame way as has previously been found for $\alpha$ elements. Finally, theelemental C/O ratios for the vast majority of our stars are below 0.8 whenusing the oxygen line at 6158A however, the forbidden oxygen line at 6300Aprovides systematically higher C/O values. Moreover, by using differentatmosphere models the C/O ratios can have a non negligible difference for coolstars. Therefore, C/O ratios should be scaled to a common solar reference inorder to correctly evaluate its behaviour.

astronomy and astrophysics

Chemical abundances of 1111 FGK stars from the HARPS GTO planet search  program IV. Carbon and C/O ratios for Galactic stellar populations and planet  hosts