  Due to the nonperturbative contribution to the hyperfine splitting the mass
of the $n^1P_1$ state is strongly correlated with the center of gravity $M_{\rm
cog}(n^3P_J)$ of the $n^3P_J$ multiplet: $M(n^1P_1)$ is less than $M_{\rm
cog}(n^3P_J)$ by about 40 MeV (20 MeV) for the 1P (2P) state. For $b_1(1235)$
the agreement with experiment is reached only if $a_0(980)$ belongs to the
$1^3P_J$ multiplet. The predicted mass of $b_1(2^1P_1)$ is $\approx 1620$ MeV.
For the isoscalar meson a correlation between the mass of $h_1$(1170)
$(h_1(1380))$ and $M_{cog}(1^3P_J)$ composed from light (strange) quarks also
takes place.
