  We consider the ground-state energy and the spectrum of the low-energy
excitations of a Majorana island formed of topological superconductors
connected by a single-mode junction of arbitrary transmission. Coulomb blockade
results in $e$-periodic modulation of the energies with the gate-induced
charge. We find the amplitude of modulation as a function of reflection
coefficient ${\cal R}$. The amplitude scales as $\sqrt{\cal R}$ in the limit
${\cal R}\to 0$. At larger ${\cal R}$, the dependence of the amplitude on the
Josephson and charging energies is similar to that of a
conventional-superconductor Cooper-pair box. The crossover value of ${\cal R}$
is small and depends on the ratio of the charging energy to superconducting
gap.
