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tokens: The low-mass inner planets of the 11 archival solar system analogues discussed in Sect. 1 are shown as white circles. It is therefore clear that to achieve higher detection frequencies for terrestrial and super-terrestrial planet at all periods, expecially shorter ones, it is necessary to obtain a higher number of observations per target star and ensure a higher-density sampling. Our observing programme, as noted in Sect. 3 , originally foresaw about 40 measurements per star, a much higher number than the average 27 per star we actually obtained; it can be argued that a number of observations closer to the one originally proposed could push the survey’s detection limits to lower planetary masses, ensuring a better coverage and higher detection frequencies for terrestrial and super-terrestrial additional planets. In fact, analytical expressions for close-in low-mass exoplanets detection threshold found in Narayan et al. (2005) show such a dependence on the number of radial velocity measurements. It will also be important in the near future comparing our results with the similarly produced detection map for the parallel HARPS-N survey mentioned in Sect. 1 . From the overall survey detection map one can finally obtain an estimate for the planetary occurence frequency f p in the presence of a giant outer planet; the probability of obtaining m detection out of a sample of size N is given by the binomial distribution: The choice of distribution is dictated by the small size of our sample (at most N = 20) and has been originally justified in Burgasser et al. (2003) and successfully used in Sozzetti et al. (2009) and Faria et al. (2016) , to name a few. During the analysis of our sample, as detailed in Sect. 4 , we found a single new candidate planet, namely the gas giant HD 50499 c, which due to its estimated minimum orbital period P ≥ 8265.51 days lies outside our considered period range of P ≥ 1 yr. Following a section of the analysis reported in the work of Mayor et al. (2011) , in which the authors estimated the occurrence frequency of stars with at leats one planet in given regions of orbital period and planetary mass, we focus our interest on sub-giant planets with orbital period less than 150 days and M sin i between 10 and 30 M ⊕ , a region for which we are conservatively sensitive to only 50% of our 20-stars sample. Having found no candidate planet falling within this mass-period space, we can only obtain a lower limit of 1− γ for the occurence frequency of low-mass planets in the presence of a known outer giant planet of f p 9.84%, a value we note to be much lower than the 38.8 ± 7.1% reported in Mayor et al. (2011) for stars hosting at least one planet in the same period-mass region. 6. Summary and discussion In this work we report the results of an intense observational campaign conducted using the high-precision HARPS spectrograph on 20 stars, obtaining an average of 27 datapoints per star. The stars in the sample were selected in