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(abridged) In the frame of the search for extrasolar planets and brown dwarfsaround early-type main-sequence stars, we present the results obtained on theearly F-type star Theta Cygni. Elodie and Sophie at OHP were used to obtain thespectra. Our dedicated radial-velocity measurement method was used to monitorthe star's radial velocities over five years. We also use complementary, highangular resolution and high-contrast images taken with PUEO at CFHT. We showthat Theta Cygni radial velocities are quasi-periodically variable, with a~150-day period. These variations are not due to the ~0.35-Msun stellarcompanion that we detected in imaging at more than 46 AU from the star. Theabsence of correlation between the bisector velocity span variations and theradial velocity variations for this 7 km/s vsini star, as well as othercriteria indicate that the observed radial velocity variations are not due tostellar spots. The observed amplitude of the bisector velocity span variationsalso seems to rule out stellar pulsations. However, we observe a peak in thebisector velocity span periodogram at the same period as the one found in theradial velocity periodogram, which indicates a probable link between theseradial velocity variations and the low amplitude lineshape variations which areof stellar origin. Long-period variations are not expected from this type ofstar to our knowledge. If a stellar origin (hence of new type) was to beconfirmed for these long-period radial velocity variations, this would haveseveral consequences on the search for planets around main-sequence stars, bothin terms of observational strategy and data analysis. An alternativeexplanation for these variable radial velocities is the presence of at leastone planet of a few Jupiter masses orbiting at less than 1 AU. (abridged)

Extrasolar planets and brown dwarfs around A-F type stars - VII. Theta Cygni radial velocity variations: planets or stellar phenomenon?