Context :

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 ( \geq 8 \sigma ) 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 \pm 20 km s ^ { -1 } ( 1 \sigma confidence ) which represents a strict lower limit to K _ { 2 } .
Also , the Doppler map of He II \lambda 4686 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 } ) \geq 0.10 M _ { \odot } .
Applying the maximum K -correction gives 228 < K _ { 2 } < 322 km s ^ { -1 } 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 .