We present a method to calculate masses for components of both eclipsing and non-eclipsing binary systems as long as their apsidal motion rates are available .
The method is based on the fact that the equation that gives the rate of apsidal motion is a supplementary equation that allows the computation of the masses of the components , if the radii and the internal structure constants of them can be obtained from theoretical models .
For this reason the use of this equation makes the method presented here model dependent .

We apply this method to calculate the mass of the components of the non-eclipsing massive binary system HD 93205 ( O3 V+O8 V ) , which is suspected to be a very young system .
To this end , we computed a grid of evolutionary models covering the mass range of interest , and taking the mass of the primary ( M _ { 1 } ) as the only independent variable , we solve the equation of apsidal motion for M _ { 1 } as a function of the age of the system .
The mass of the primary we find ranges from M _ { 1 } = 60 \pm 19 { M } _ { \odot } for ZAMS models , which sets an upper limit for M _ { 1 } , down to M _ { 1 } = 40 \pm 9 { M } _ { \odot } for an age of 2 Myr .
Accordingly , the upper limit derived for the mass of the secondary ( M _ { 2 } = QM _ { 1 } ) is M _ { 2 } = 25 { M } _ { \odot } is in very good agreement with the masses derived for other O8 V stars occurring in eclipsing binaries .