Parameter space for the detectability of longer period planets in the systems HD 102956 (left), 8 Umi (middle), and 70 Vir (right), assuming circular orbits. Planets located in the red shaded region are not detectable by the RV data obtained by Johnson et al. (2010), Lee et al. (2015), and Naef et al. (2003), respectively. The blue dashed line indicates the planet mass that would produce a RV amplitude equal to the standard deviation of the RV data (once the signal of the inner planet has been removed). See details on the calculation in Sect. 3.