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(Abridged) Aims: Systematic surveys to search for exoplanets have been mostlydedicated to solar-type stars sofar. We developed in 2004 a method to extendsuch searches to earlier A-F type dwarfs and started spectroscopic surveys tosearch for planets and quantify the detection limit achievable when taking intoaccount the stars properties and their actual levels of intrinsic variations.We give here the first results of our southern survey with HARPS.  Results: 1) 64% of the 170 stars with enough data points are found to bevariable. 20 are found to be binaries or candidate binaries (with stars orbrown dwarfs). More than 80% or the latest type stars (once binaries areremoved) are intrinsically variable at a 2 m/s precision level. Stars withearlier spectral type (B-V <= 0.2) are either variable or associated to levelsof uncertainties comparable to the RV rms observed on variable stars of sameB-V. 2) We have detected one long-period planetary system around an F6IV-Vstar. 3) We have quantified the jitter due to stellar activity and we show thattaking into account this jitter in addition to the stellar parameters, it isstill possible to detect planets with HARPS with periods of 3 days (resp. 10days and 100 days) on 91% (resp. 83%, 61%) of them. We show that even theearliest spectral type stars are accessible to this type of search, providedthey have a low vsini and low levels of activity. 4) Taking into account thepresent data, we compute the actually achieved detection limits for 107 targetsand discuss the limits as a function of B-V. Given the data at hand, our surveyis sensitive to short-period (few days) planets and to longer ones (100 days)at a lower extent (latest type stars). We derive first constrains on thepresence of planets around A-F stars for these ranges of periods.

Extrasolar planets and brown dwarfs around A-F type stars VI. High precision RV survey of early type dwarfs with HARPS