We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii , the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4 - 3658 , obtained during the 2008 September/October outburst .
Doppler tomography of the N iii \lambda 4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star .
The velocity of this emission occurs at 324 \pm 15 km s ^ { -1 } ; applying a “ K -correction ” , we find the velocity of the secondary star projected onto the line of sight to be 370 \pm 40 km s ^ { -1 } .
Based on existing pulse timing measurements , this constrains the mass ratio of the system to be 0.044 ^ { +0.005 } _ { -0.004 } , and the mass function for the pulsar to be 0.44 ^ { +0.16 } _ { -0.13 } M _ { \sun } .
Combining this mass function with various inclination estimates from other authors , we find no evidence to suggest that the neutron star in SAX J1808.4 - 3658 is more massive than the canonical value of 1.4 M _ { \sun } .
Our optical light curves exhibit a possible superhump modulation , expected for a system with such a low mass ratio .
The equivalent width of the Ca ii H and K interstellar absorption lines suggest that the distance to the source is \sim 2.5 kpc .
This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen , but lower than more recent estimates which assume helium-rich bursts .