  Observations of the accretion powered millisecond pulsar SAX J1808.4-3658
have revealed an interesting binary evolution, with the orbit of the system
expanding at an accelerated rate. We use the recent finding that the accreted
fuel in SAX J1808.4-3658 is hydrogen depleted to greatly refine models of the
progenitor and prior evolution of the binary system. We constrain the initial
mass of the companion star to 1.0-1.2 M$_{\mathrm{\odot}}$, more massive than
previous evolutionary studies of this system have assumed. We also infer the
system must have undergone strongly non-conservative mass transfer in order to
explain the observed orbital period changes. Following Jia & Li (2015), we
include mass loss due to the pulsar radiation pressure on the donor star,
inducing an evaporative wind which is ejected at the inner Lagrangian point of
the binary system. The resulting additional loss of angular momentum resolves
the discrepancy between conservative mass transfer models and the observed
orbital period derivative of this system. We also include a treatment of donor
irradiation due to the accretion luminosity, and find this has a non-negligible
effect on the evolution of the system.
