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

International audienceAims: In the framework of the search for extrasolar planets and brown dwarfs around early-type main-sequence stars, we present the results obtained on the early F-type star θ Cygni. Methods: ELODIE and SOPHIE at the Observatoire de Haute-Provence (OHP) were used to obtain 91 and 162 spectra, respectively. Our dedicated radial-velocity measurement method was used to monitor the star's radial velocities over five years. We also used complementary, high angular resolution and high-contrast images taken with PUEO at the CFHT. Results: We show that θ Cygni radial velocities are quasi-periodically variable, with a ≃150-day period. These variations are not due to the ≃0.35-Mȯ stellar companion that we detected in imaging at more than 46 AU from the star. The absence of correlation between the bisector velocity span variations and the radial velocity variations for this 7 km s^-1 v sin i star, as well as other criteria, indicate that the observed radial velocity variations do not stem from stellar spots. The observed amplitude of the bisector velocity span variations also seems to rule out stellar pulsations. However, we observe a peak in the bisector velocity span periodogram at the same period as the one found in the radial velocity periodogram, which indicates a probable link between these radial velocity variations and the low-amplitude lineshape variations with a stellar origin. Long-period variations are not expected from this type of star to our knowledge. If a stellar origin (hence of new type) were to be confirmed for these long-period radial velocity variations, this would have several consequences on the search for planets around main-sequence stars, both in terms of observational strategy and data analysis. An alternative explanation for these variable radial velocities is the presence of at least one planet of a few Jupiter masses orbiting at less than 1 AU; however, this planet alone does not explain all observed features, and the θ Cygni system is obviously more complex than a planetary system with 1 or 2 planets. Conclusions: The available data do not allow us to distinguish between these two possible origins. A vigourous follow-up in spectroscopy and photometry is needed to get a comprehensive view of the star intrinsic variability and/or its surrounding planetary system. Based on observations made with the ELODIE and SOPHIE spectrographs at the Observatoire de Haute-Provence (CNRS, France) and with the PUEO adaptive optics system at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Tables of radial velocities are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/146