  The data collected with a radioactively pure ZnWO$_4$ crystal scintillator
(699 g) in low background measurements during 2130 h at the underground (3600 m
w.e.) Laboratori Nazionali del Gran Sasso (INFN, Italy) were used to set a
limit on possible concentration of superheavy eka-W (seaborgium Sg, Z = 106) in
the crystal. Assuming that one of the daughters in a chain of decays of the
initial Sg nucleus decays with emission of high energy $\alpha$ particle
($Q_\alpha > 8$ MeV) and analyzing the high energy part of the measured
$\alpha$ spectrum, the limit N(Sg)/N(W) < 5.5 $\times$ 10$^{-14}$ atoms/atom at
90% C.L. was obtained (for Sg half-life of 10$^9$ yr). In addition, a limit on
the concentration of eka-Bi was set by analysing the data collected with a
large BGO scintillation bolometer in an experiment performed by another group
[L. Cardani et al., JINST 7 (2012) P10022]: N(eka-Bi)/N(Bi) < 1.1 $\times$
10$^{-13}$ atoms/atom with 90% C.L. Both the limits are comparable with those
obtained in recent experiments which instead look for spontaneous fission of
superheavy elements or use the accelerator mass spectrometry.
