  The limitations of the doublet-septet mixing models by the deviations of
electroweak oblique parameters $\Delta S$ and $\Delta T$ are studied. In the
minimal model, the mixture of the septet $\eta$ and the scalar doublet in the
standard model (SM) is driven by a non-Hermitian dimension-7 operator. For a
smaller bare mass of the septet, $\Delta S$ gives a stringent constraint on the
mixing angle $\sin\beta$ between the CP-odd neutral parts of the SM Higgs
doublet and $\eta$. In general, increasing the mass of the scalar septet
$M_\eta$ will enhance the deviation of $T$ from the SM, whereas it decreases
the magnitude of $\Delta S$ for a larger bare mass within the range
$M_\eta\lesssim 400\,{\rm GeV}$. We also examine two extended models from the
ordinary doublet-septet mixture pattern. One of them is based on a inert
doublet-septet mixing pattern, in which there is no vacuum expectation value
for the neutral component of $\eta$, and a stable dark matter could naturally
exist. For a benchmark point with this inner doublet mass of $M_\chi=250{\rm}$
and $M_\eta=400\,{\rm GeV}$ in this model, the mixing coefficient is found to
be less than $1.8$. The other extension is constructed by imposing a doubly
charged scalar mixed with the doubly charged component of the septet. Apart
from the contribution by the septet-doublet admixture, $\Delta S$ is suppressed
by a factor of $s_W^2$ and $\Delta T$ has a significant constraint due to the
vanishing vacuum polarization of $Z$ at the momentum transfer $p^2=0$.
