Constraints on the Neutron Star Magnetic Field of the two X-ray Transients SAX J1808.4-3658 and Aql X-1

The recently discovered coherent X-ray pulsations at a frequency of ~400 Hz in SAX J1808.4-3658, together with a measure of the source luminosity in quiescence, allow us to put an upper limit on the neutron star magnetic field, that is B < 5 10^8 Gauss, using simple considerations on the position of the magnetospheric radius during quiescent periods. Combined with the lower limit inferred from the presence of X-ray pulsations, this constrains the SAX J1808.4-3658 neutron star magnetic field in the quite narrow range (1-5) 10^8 Gauss. Similar considerations applied to the case of Aql X-1 give a neutron star magnetic field lower than ~10^9 Gauss.