We analyzed 123 thermonuclear ( type-I ) X-ray bursts observed by the Rossi X-ray Timing Explorer from the low-mass X-ray binary 4U 1636 - 536 .
All but two of the 40 radius-exansion bursts in this sample reached peak fluxes which were normally distributed about a mean of 6.4 \times 10 ^ { -8 } { ergs cm ^ { -2 } s ^ { -1 } } , with a standard deviation of 7.6 % .
The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 \pm 0.13 lower than this mean value ; as a consequence , the overall variation in the peak flux of the radius-expansion bursts was a factor of \approx 2 .

This variation is comparable to the range of the Eddington limit between material with solar H-fraction ( X = 0.7 ) and pure He .
Such a variation may arise if , for the bright radius-expansion bursts , most of the accreted H is eliminated either by steady hot CNO burning or expelled in a radiatively-driven wind .
However , steady burning can not exhaust the accreted H for solar composition material within the typical \approx 2 hr burst recurrence time , nor can it result in sufficient elemental stratification to allow selective ejection of the H only .
An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen .
We also observed non-radius expansion bursts that exceeded the peak flux of the faintest radius expansion bursts .
For these bursts the accreted hydrogen must have been partly ejected or eliminated , but the burst flux did not subsequently reach the ( higher ) Eddington limit for the underlying He-rich material .

We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius expansion bursts , previously observed in smaller samples .
Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material ( X \approx 0.7 ) , and the brighter bursts the limit for pure He ( X = 0 ) , we estimate the distance to 4U 1636 - 536 ( for a canonical neutron star with M _ { NS } = 1.4 M _ { \odot } , R _ { NS } = 10 km ) to be 6.0 \pm 0.5 kpc , or for M _ { NS } = 2 M _ { \odot } at most 7.1 kpc .