Absolute r-band magnitude of the BGG (top panel) and X-ray luminosity of the system (bottom panel) as a function of the gap in magnitude between the two brightest galaxies of the systems studied in this paper (open red circles) and in Aguerri et al. (2007, open blue circles). The green filled circles are the genuine fossil systems. The points with a right arrow are those systems for which the magnitude gap is a lower limit.