  Masses of tetraquarks with two heavy quarks and open charm and bottom are
calculated in the framework of the diquark-antidiquark picture in the
relativistic quark model. All model parameters were regarded as fixed by
previous considerations of various properties of mesons and baryons. The light
quarks and diquarks are treated completely relativistically. The c quark is
assumed to be heavy enough to make the diquark configurations dominating. The
diquarks are considered not to be point-like but to have an internal structure
which is taken into account by the calculated diquark form factor entering the
diquark-gluon interaction. It is found that all the (cc)(\bar q\bar q')
tetraquarks have masses above the thresholds for decays into open charm mesons.
Only the I(J^P)=0(1^+) state of (bb)(\bar u\bar d) lies below the BB* threshold
and is predicted to be narrow.
