  The only prominent line of singly ionized helium in the visible spectral
range, helium-II 4686 A, is observed together with the helium-I 5015 A singlet
and the helium-I 4471 A triplet line in solar prominences. The sodium emission,
NaD2, is used as a tracer for helium-II emissions which are sufficiently bright
to exceed the noise level near 10^-6 of the disk-center intensity. The so
selected prominences are characterized by small non-thermal line broadening and
almost absent velocity shifts, yielding narrow line profiles without wiggles.
The reduced widths [Delta(lambda_D) / lambda] of helium-II 4686 A are 1.5 times
broader than those of helium-I 4471 A triplet and 1.65 times broader than those
of helium-I 5015 A singlet. This indicates that the helium lines originate in a
prominence--corona transition region with outwards increasing temperature.
