We present a model for determining the mass ratio in interacting binaries by directly fitting the observed spectrum with synthetic spectra .
We make direct use of \sc NextGen model atmospheres intensities which are the most comprehensive and detailed models available for cool stars .
We fully take into account the varying temperature and gravity across the secondary star ’ s photosphere , by incorporating the synthetic spectra into the secondary star ’ s Roche geometry .
As a result , we determine the exact rotationally broadened spectrum of the secondary star and so eliminate the need for a limb-darkening law , and the uncertainties associated with it .

As an example we determine the mass ratio for the well studied soft X-ray transient Nova Sco 1994 .
In order to obtain a more accurate determination of the mass ratio , which does not depend on assumptions about the rotation profile and limb-darkening coefficients , we use our model to compute the exact rotationally broadened model spectrum , which we compare directly with the observed intermediate resolution spectrum of Nova Sco 1994 .
We determine the mass ratio of Nova Sco 1994 to be 0.419 \pm 0.028 ( 90 percent confidence ) , which is the most accurate determination of the binary mass ratio in an X-ray binary .
This result combined with the binary mass function and inclination angle gives a refined black hole mass of 5.99 \pm 0.42 M _ { \odot } ( 90 percent confidence ) .
We also perform simulations which show that , for an F-type secondary star , the standard rotation profile with zero and continuum value for the line limb-darkening coefficient gives a value for q that brackets the value found using the full geometrical treatment