IUE observations of two extremes among contact binaries: AW Ursae Majoris and SW Lacertae

We present IUE observations of AW UMa and SW Lac. These stars are the most extreme representatives of the A- and W-types of W UMa stars and might therefore be expected to differ most in their properties. We find that the stars are similar in their transition region emission (N V, C IV, Si IV) but differ markedly in their chromospheric emission (Mg II). In SW Lac the strength of the He II line is consistent with current understanding of its excitation by the coronal X-ray flux (Hartmann et al.). In AW UMa the He II strength is larger than might be anticipated given the low upper limit on the X-ray emission of this star. We obtain an upper limit to the phase dependence of the transition region lines in AW UMa. (author)