SFXT window in P_orb − e plane. The region filled with small  +  denotes the prohibited region where binaries cannot stay in stable orbits. Systems in the right-lower region filled with black dots produce no X-ray flares, since R_acc < R_cl through the orbital motion (case A). The left-lower shaded part indicates the region where R_acc > R_cl through the orbital motion (case B). Systems in this region will show persistent outburst. The central-upper region denotes SFXT window characterized by quiescence and outburst (case C). Filled circles show the position of the observed SFXTs; (a) IGR J16479-4515, (b) IGR J17544-2619, (c) IGR J18483-0311, (d) SAX J1818.6-1703, (e) IGR J16465-4507, (f) XTE J1739-302. The orbital period of (g) IGR J11215-5952 is 165 [d] and this object is situated far outside of this figure. The filled square indicates observed classical HMXBs; SAX J1802.7-201 (P_orb = 4.6[d], e < 0.2), Vela-X1 (P_orb = 9[d], e = 0.1) and GX301-2 (P_orb = 41.5[d], e = 0.47).