The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4 - 3658 during 1998–2008 .
We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions .
The spin frequency of the source continues to decrease at a rate of ( -5.5 \pm 1.2 ) \times 10 ^ { -18 } Hz s ^ { -1 } , which is consistent with the previously determined spin derivative .
The spin-down occurs mostly during quiescence , and it is most likely due to the magnetic dipole torque from a B = 1.5 \times 10 ^ { 8 } G dipolar field at the neutron star surface .
We also find that the 2 hr binary orbital period is increasing at a rate of ( 3.80 \pm 0.06 ) \times 10 ^ { -12 } s s ^ { -1 } , also consistent with previous measurements .
It remains uncertain whether this orbital change reflects secular evolution or short-term variability .