  For the study of the $\Sigma$ resonances, we analyze the differential cross
sections and $\Lambda$ polarizations for the reactions $K^-n\to\pi^-\Lambda$
and $K^-p\to\pi^0\Lambda$ with an effective Lagrangian approach. Data of an
early experiment and the recent Crystal Ball experiment at BNL are included in
the analysis with the c.m. energy from 1550 to 1676 MeV. Our results clearly
support the existence of a $\Sigma$ resonance with $J^P={1\over 2}^+$, mass
near 1633 MeV, and width about 120 MeV, which confirms the 3-star $\Sigma(1660)
1/2^+$ in PDG. Meanwhile, our results do not support the existence of the
2-star $\Sigma(1620){1\over 2}^-$ in PDG. The analysis results for the
parameters of the relevant $\Sigma$ resonances and couplings are presented.
