  Aims. This paper reports accurate laboratory frequencies of the rotational
ground state transitions of two astronomically relevant molecular ions, NH3D+
and CF+. Methods. Spectra in the millimeter-wave band were recorded by the
method of rotational state-selective attachment of He-atoms to the molecular
ions stored and cooled in a cryogenic ion trap held at 4 K. The lowest
rotational transition in the A state (ortho state) of NH$_3$D$^+$ ($J_K = 1_0 -
0_0$), and the two hyperfine components of the ground state transition of
CF$^+$($J = 1 - 0$) were measured with a relative precision better than
$10^{-7}$. Results. For both target ions the experimental transition
frequencies agree with recent observations of the same lines in different
astronomical environments. In the case of NH$_3$D$^+$ the high-accuracy
laboratory measurements lend support to its tentative identification in the
interstellar medium. For CF$^+$ the experimentally determined hyperfine
splitting confirms previous quantum-chemical calculations and the intrinsic
spectroscopic nature of a double-peaked line profile observed in the $J = 1 -
0$ transition towards the Horsehead PDR.
