W UMa type binaries have two defining characteristics .
These are ( i ) the effective temperatures of both components are very similar , and ( ii ) the secondary ( currently less massive ) component is overluminous for its current mass .
We consider the latter to be an indication of its mass before the mass-transfer event .
For these stars , we define a mass difference ( \delta M ) between the mass determined from its luminosity and the present mass determined from fitting the binary orbit .
We compare the observed values of the mass difference to stellar models with mass-loss .
The range of initial secondary masses that we find for observed W UMa type binaries is 1.3 - 2.6 \mathrm { M } _ { \odot } .
We discover that the A- and the W-subtype contact binaries have different ranges of initial secondary masses .
Binary systems with an initial mass higher than 1.8 \pm 0.1 \mathrm { M } _ { \odot } become A-subtype while systems with initial masses lower than this become W-subtype .
Only 6 per cent of systems violate this behavior .
We also obtain the initial masses of the primaries using the following constraint for the reciprocal of the initial mass ratio : 0 < 1 / q _ { i } < 1 .
The range of initial masses we find for the primaries is 0.2 - 1.5 \mathrm { M } _ { \odot } , except for two systems .
Finally in comparing our models to observed systems , we find evidence that the mass transfer process is not conservative .
We find that only 34 per cent of the mass from the secondary is transferred to the primary .
The remainder is lost from the system .