Precision observables in SU(2) x U(1) models with an additional Higgs triplet

Electroweak precision observables are calculated at complete 1-loop order in the extension of the standard model by an extra Higgs triplet, where the $\rho$-parameter can be different from unity already at the tree level. One additional data point is required for fixing the input parameters. In the on-shell renormalization scheme the leptonic mixing angle $\sin^2\theta_e$ at the $Z$ peak is chosen, together with the conventional input $\alpha, M_Z, G_{\mu}, m_t$. The calculated observables depend on the mass of the doublet Higgs boson $H^0$ and on the masses of the extra non-standard Higgs bosons as free parameters. The predictions of the standard model and the triplet model coincide for all observables in the experimental range of the top mass $m_t=175\pm 6$ GeV. In the triplet model, all observables which show a dependence on the doublet Higgs mass $M_{H^0}$ are consistent with a low value of $M_{H^0}$.