  aims: We obtained phase-resolved spectroscopy of the accreting millisecond
X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature
of the donor star, constrain its radial velocity semi-amplitude (K_2), and
derive estimates on the pulsar mass. methods: Using Doppler images of the Bowen
region we find a significant (>8sigma) compact spot at a position where the
donor star is expected. If this is a signature of the donor star, we measure
K_em=248+/-20 km/s (1sigma confidence) which represents a strict lower limit to
K_2. Also, the Doppler map of He II lambda4686 shows the characteristic
signature of the accretion disk, and there is a hint of enhanced emission that
may be a result of tidal distortions in the accretion disk that are expected in
very low mass ratio interacting binaries. results: The lower-limit on K_2 leads
to a lower-limit on the mass function of f(M_1)>0.10M_sun. Applying the maximum
K-correction gives 228<K_2<322 km/s and a mass ratio of 0.051<q<0.072.
conclusions: Despite the limited S/N of the data we were able to detect a
signature of the donor star in SAX J1808.4-3658, although future observations
during a new outburst are still warranted to confirm this. If the derived K_em
is correct, the largest uncertainty in the determination of the mass of the
neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly
known inclination.
