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Highly charged ions for atomic clocks and search for variation of the fine structure constant

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, $α$ ($α= e^2/\hbar c$). E.g., californium and einsteinium ions have largest known sensitivity to $α$-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.