  Excited states in the well-deformed rare earth isotopes $^{154}$Sm and
$^{166}$Er were populated via ``safe'' Coulomb excitation at the Munich MLL
Tandem accelerator. Conversion electrons were registered in a cooled Si(Li)
detector in conjunction with a magnetic transport and filter system, the
Mini-Orange spectrometer. For the first excited $0^+$ state in $^{154}$Sm at
1099 keV a large value of the monopole strength for the transition to the
ground state of $\rho^2(\text{E0}; 0^+_2 \to 0^+_\text{g}) = 96(42)\cdot
10^{-3}$ could be extracted. This confirms the interpretation of the lowest
excited $0^+$ state in $^{154}$Sm as the collective $\beta$-vibrational
excitation of the ground state. In $^{166}$Er the measured large electric
monopole strength of $\rho^2(\text{E0}; 0^+_4 \to 0^+_1) = 127(60)\cdot
10^{-3}$ clearly identifies the $0_4^+$ state at 1934 keV to be the
$\beta$-vibrational excitation of the ground state.
