  We report on the detection of a series of six lines in the ultra-deep Q-band
integration toward IRC+10216 carried out with the Yebes 40m telescope, which
are in harmonic relation with integer quantum numbers J from 12 to 18. After a
detailed analysis of all possible carriers, guided by high-level quantum
chemical calculations, we conclude that the lines belong to HMgCCCN, named
hydromagnesium cyanoacetylide. The rotational temperature and column density
derived for HMgCCCN are 17.1 +/- 2.8K and (3.0 +/- 0.6) e12 cm-2, respectively.
The observed abundance ratio between MgCCCN and HMgCCCN is 3. In addition, we
report the discovery in space, also toward IRC+10216, of sodium cyanoacetylide,
NaCCCN, for which accurate laboratory data are available. For this species we
derive a rotational temperature of 13.5 +/- 1.7K and a column density of (1.2
+/- 0.2) e11 cm-2.
