  A quantitative characterization of the relationship between molecular
sequence and structure is essential to improve our understanding of how
function emerges. This particular genotype-phenotype map has been often studied
in the context of RNA sequences, with the folded configurations standing as a
proxy for the phenotype. Here, we count the secondary structures of circular
RNAs of length $n$ and calculate the asymptotic distributions of different
structural moieties, such as stems or hairpin loops, by means of symbolic
combinatorics. Circular RNAs differ in essential ways from their linear
counterparts. From the mathematical viewpoint, the enumeration of the
corresponding secondary structures demands the use of combinatorial techniques
additional to those used for linear RNAs. The asymptotic number of secondary
structures for circular RNAs grows as $a^nn^{-5/2}$, with a depending on
particular constraints applied to the secondary structure. The abundance of any
structural moiety is normally distributed in the limit $n\to\infty$, with a
mean and a variance that increase linearly with $n$.
