The spin-orbital period plane of confirmed IPs (crosses) including the sample of the nine sources (filled circles). The ranges of P_Ω for WX Pyx and IGR J1817 and the lower limit for RX J0636 are also reported. P_ω/P_Ω = 0.01, 0.1 and 1. are reported as solid lines. The orbital period gap is shown as vertical dashed lines. The spin (right panel) and the orbital (upper panel) period distributions of the whole sample are also shown. Confirmed IPs are taken from http://asd.gsfc.nasa.gov/Koji.Mukai/iphome/iphome.html, and we also include RX J0944.5+0357 (de Martino 2007), IGR J19267+1325 (Evans et al. 2008), RX J052430.2+4244 (Schwarz et al. 2007) and IGR J16544-1916 (Scaringi et al. 2011).