  Our observations of TMC-1 with the Yebes 40 m radio telescope in the
31.0-50.3 GHz range allowed us to detect a group of unidentified lines, showing
a complex line pattern indicative of an open-shell species. {}The observed
frequencies of these lines and the similarity of the spectral pattern with that
of the 2$_{0,2}$-1$_{0,1}$ rotational transition of H$_2$CCN indicate that the
lines arise from the deuterated cyanomethyl radical, HDCCN. Using Fourier
transform microwave spectroscopy experiments combined with electric discharges,
we succeeded in producing the radical HDCCN in the laboratory and observed its
1$_{0,1}$-0$_{0,0}$ and 2$_{0,2}$-1$_{0,1}$ rotational transitions. From our
observations and assuming a rotational temperature of 5 K, we derive an
abundance ratio H$_2$CCN/HDCCN=20$\pm$4. The high abundance of the deuterated
form of H$_2$CCN is well accounted for by a standard gas-phase model, in which
deuteration is driven by deuteron transfer from the H$_2$D$^+$ molecular ion.
