chance to overcome the latter issue. Also, now that recent laboratory studies of nitrogen hydrides in their various deuterated forms have been reported up to the terahertz domain [@melosso2017terahertz; @melosso2019rotational; @melosso2019pure; @melosso2020nh2d; @bizzocchi2020nhd], confident searches for these species can be undertaken in *Herschel* archival surveys.

In this paper, we report the first detection of and in the interstellar medium, thanks to HIFI[^1] observations carried out as part of the CHESS[^2] key program (PI: C. Ceccarelli). Both species were observed in absorption against the continuum of the low-luminosity protostar IRAS 16293-2422. From the modeling of their hyperfine structures, column densities of and have been determined and the deuterium fractionation of amidogen radical evaluated.

Observations
============

The observations analysed in the present paper have been retrieved from the HSA[^3]. They were collected as part of CHESS key program, whose observational details have been given extensively in @ceccarelli2010herschel. Briefly, the Class 0 protostar IRAS 16293-2422 was observed in March 2010 with the HIFI spectrometer [@degraauw2010hifi; @roelf2012hifi] for a total of 50h. The targeted coordinates were $\alpha_{2000} = {\ensuremath{{16}^{\ensuremath{\mathrm{h}}}{23}^{\ensuremath{\mathrm{m}}}{22.75}^{\ensuremath{\mathrm{s}}}}\xspace}$, $\delta_{2000} = {\ensuremath{-24^\circ 28^\prime 34.2^{\prime\prime}}\xspace}$. Seven different HIFI bands were employed to perform a spectral survey covering the frequency range between 480 and 1902GHz almost 