We analysed Rossi X-ray Timing Explorer observations of the accretion-powered 401 Hz pulsar SAX J1808.4 - 3658 , in order to precisely determine the source distance .
While the fluences for the five transient outbursts observed from 1996 were constant to within the uncertainties , the outburst interval varied signficantly , so that the time-averaged flux ( and accretion rate ) decreased by around 40 % .
By equating the time-averaged X-ray flux with the expected mass transfer rate from gravitational radiation , we derived a lower limit on the distance of 3.4 kpc .
Combined with an upper limit from assuming that the four radius-expansion thermonuclear bursts observed during the 2002 October outburst reached at most the Eddington limit for a pure He atmosphere , we found that the probable distance range for the source is 3.4–3.6 kpc .
The implied inclination , based on the optical/IR properties of the counterpart , is i \lesssim 30 ^ { \circ } .

We compared the properties of the bursts with an ignition model .
The time between bursts was long enough for hot CNO burning to significantly deplete the accreted hydrogen , so that ignition occurred in a pure helium layer underlying a stable hydrogen burning shell .
This is the first time that this burning regime has been securely observationally identified .
The observed energetics of the bursts give a mean hydrogen fraction at ignition of \left < X \right > \approx 0.1 , and require that the accreted hydrogen fraction X _ { 0 } and the CNO metallicity Z _ { CNO } are related by Z _ { CNO } \approx 0.03 ( X _ { 0 } / 0.7 ) ^ { 2 } .
We show that in this burning regime , a measurement of the burst recurrence time and energetics allows the local accretion rate onto the star to be determined independently of the accreted composition , giving a new method for estimating the source distance which is in good agreement with our other estimates .