  An evolutionary scenario to explain the transient nature and short total
duration of the X-ray burst of SAX J1808.4 -- 3658 is proposed. An optical
companion of the neutron star (a ``turn-off'' Main - Sequence star) fills its
Roche lobe at the orbital period ($P_{orb}$) $\sim$ 19 hours. During the
initial high mass--transfer phase when the neutron star is a persistent X-ray
source, the neutron star is spun up to a millisecond period. Due to its
chemical composition gradient, the secondary does not become fully convective
when its mass decreases below 0.3 $\msun$, hence a magnetic braking remains an
effective mechanism to remove orbital angular momentum and the system evolves
with Roche - lobe overflow towards a short orbital period. Near an orbital
period of two hours the mass transfer rate becomes so small ($\sim$
$10^{-11}\msun$/yr) that the system can not continue to be observed as a
persistent X-ray source. During further Roche - lobe filling evolution deep
mixing allows the surface of secondary to become more and more helium rich.
Since the accreted matter is helium rich, it is easy to explain observed short
total duration of the burst . This evolutionary picture suggest that radio
emission can be observed only at shorter wavelength's. Our model predicts a
faster orbital period decay than expected if the orbital evolution is driven
only by gravitational wave radiation.
