  We present the results of an experimental and theoretical study of the
electronically excited $\tripletex$ state of $^{87}$Rb$_2$ molecules. The
vibrational energies are measured for deeply bound states from the bottom up to
$v'=15$ using laser spectroscopy of ultracold Rb$_2$ Feshbach molecules. The
spectrum of each vibrational state is dominated by a 47\,GHz splitting into a
$\cog$ and $\clg$ component caused mainly by a strong second order spin-orbit
interaction. Our spectroscopy fully resolves the rotational, hyperfine, and
Zeeman structure of the spectrum. We are able to describe to first order this
structure using a simplified effective Hamiltonian.
