From the extreme position of disk motion, we infer the neutron star (NS) surface magnetic field strength (B _0) of Z-source GX 17+2 and Cyg X-2. The inferred B _0 of GX 17+2 and Cyg X-2 are ~(1-5)×10~8 G and ~(1-3)×10~8 G, respectively, which are not inferior to that of millisecond X-ray pulsars or atoll sources. It is likely that the NS magnetic axis of Z sources is parallel to the axis of rotation, which could result in the lack of pulsations in these sources.
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