  We review a number of highly charged ions which have optical transitions
suitable for building extremely accurate atomic clocks. This includes ions from
Hf$^{12+}$ to U$^{34+}$, which have the $4f^{12}$ configuration of valence
electrons, the Ir$^{17+}$ ion, which has a hole in almost filled $4f$ subshell,
the Ho$^{14+}$, Cf$^{15+}$, Es$^{17+}$ and Es$^{16+}$ ions. Clock transitions
in most of these ions are sensitive to variation of the fine structure
constant, $\alpha$ ($\alpha = e^2/\hbar c$). E.g., californium and einsteinium
ions have largest known sensitivity to $\alpha$-variation while holmium ion
looks as the most suitable ion for experimental study. We study the spectra of
the ions and their features relevant to the use as frequency standards.
