  A description of eigensubspaces of the cosine and sine operators is
presented.
  The spectrum of each of these two operator consists of two eigenvalues
(1,\,-1) and their eigensubspaces are infinite--dimensional. There are many
possible bases for these subspaces, but most popular are bases constructed from
the Hermite functions. We present other "bases" which are not discrete
orthogonal sequences of vectors, but continuous orthogonal chains of vectors.
Our work can be considered a continuation and further development of results in
\textit{Self-reciprocal functions} by Hardy and Titchmarsh:
  Quarterly Journ. of Math. (Oxford Ser.) \textbf{1} (1930).
