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

On the origin of W UMa type Contact binaries - a new method for computation of initial masses