The pseudoscalar meson B_c is the lowest mass bound state of the (c b-bar) and the charge conjugate. It is the last such meson to be discovered. Both the CDF and D0 collaborations have reported recent results on the mass and lifetime of this elusive state.
@article{arxiv.hep-ex/0506061,
title = {Recent Results on the Bc Meson},
author = {M. D. Corcoran},
journal= {arXiv preprint arXiv:hep-ex/0506061},
year = {2007}
}