  We present observations of VUV transitions between doubly excited sextet
states in O IV, F V and Ne VI. Spectra were produced by collisions of an O+
beam with a solid carbon target. We also studied spectra obtained previously of
F V and Ne VI. Some observed lines were assigned to the 1s2s2p23d 6L - 1s2p33d
6D, L=F, D, P electric-dipole transitions, and compared with results of MCHF
(with QED and higher-order corrections) and MCDF calculations. 42 new lines
have been identified. Highly excited sextet states in five-electron ions
provide a new form of energy storage and are possible candidates for VUV and
x-ray lasers.
