  We report the detection of the protonated form of HC7N in TMC-1. The
discovery of the cation HC7NH+ was carried out via the observation of nine
harmonically related lines in the Q-band using the Yebes 40m radiotelescope.
The observed frequencies allowed us to obtain the rotational constants
B_0=553.938802(160)MHz and D_0=3.6292(705) Hz. The identification of HC7NH+ is
further supported by accurate ab initio calculations. We derived a column
density of (5.5+/-0.7)e10 cm-2, which constitutes another piece of evidence for
the identification of the carrier. In addition, we revised the HC7N column
density and we derived a new value of (2.1+/-0.2)e13cm-2. Hence, the abundance
ratio HC7N/HC7NH+ is 380, while those for HC3N/HC3NH+ and HC5N/HC5NH+ are 230
and 240, respectively. Here, we discuss these results within the framework of a
chemical model for protonated molecules in cold dense clouds.
