A tale of two Hardy spaces

New relations between the Fourier transform of a function of bounded
variation and the Hilbert transform of its derivative are revealed.
The main result is an asymptotic formula for the {\bf cosine} Fourier
transform. Such relations have previously been known only for the sine
Fourier transform. Interrelations of various function spaces are studied
in this context, first of all of two types of Hardy spaces. The obtained
results are used for proving completely new results on the integrability
of trigonometric series.