Investigations on the structure of W UMa stars.

Starting with the rectffiable model and the Roche model for W UMa stars it is shown that the photometric elements depend remarkably on the rectffication procedure. Taking into account this effect the photometric elements for 34 W UMa systems are recalculated and a catalogne of preliminary elements is given. On the basis of these results, the model of Lucy (1968) for W UMa stars is tested. It is shown that the theoretically expected colour-mass ratio-relation is indicated; however, the postulation of a common convective envelope in the meaning of Lucy's model is in contradiction to the observations. Nevertheless it is shown that a small contact of both components cannot be excluded. Whether the components can be regarded as main sequence stars can be proved only after a theoretical calculation of the anomalous behaviour of the luminosities with respect to possible contact. From an estimate of the reflection effect it follows that this effect is not sufficient for an explanation of the reddening in the minima. A discussion of the arguments leads to the conclusion that the W UMa stars very probably are contact systems. Key words: eclipsing binaries - contact systems - photometric elements