  We report the detection of two lines at millimetre wavelengths towards the
immediate surroundings of the post-giant and most likely post-common-envelope
star HD101584 using high-angular-resolution ALMA observations. The
circumstellar environment of this object is rich in different molecular
species, but we find no viable identifications in terms of molecular lines.
  We aim to determine whether or not these lines can be attributed to the
Rydberg transitions -- X30alpha and X26alpha -- of neutral atoms of elements
heavier than carbon.
  A simple model in strict local thermodynamic equilibrium for a warm-gas
environment of the moderate-temperature star (T_eff about 8500 K) was
constructed to corroborate our findings. A geometrically thin, disc-like
geometry seen face-on was chosen and a distance of 1 kpc.
  The observed flux densities of the lines and the continuum at 232 and 354 GHz
can be reproduced using 10^(-3) M_sun of gas at a temperature of about 2800 K
and a hydrogen density of about 10^(12) cm(-3), assuming solar abundances for
the elements. The gas lies within a distance of about 5 au from the star
(assuming a distance of 1 kpc). The ionisation fraction is low, about
3x10^(-5). The origin of such a region is not clear, but it may be related to a
common-envelope-evolution phase. With these conditions, the line emissions are
dominated by Rydberg transitions within the stable isotopes of Mg. A turbulent
velocity field in the range 5.5 - 7.5 km s^(-1) is required to fit the Gaussian
line shapes. An upper limit to the average magnetic field in the line-emitting
region of 1G is set using the Zeeman effect in these lines.
  We speculate that Rydberg transitions of heavy elements may be an interesting
probe for the close-in environments of other moderate-temperature objects like
AGB stars, red supergiants, yellow hypergiants, and binaries of various types.
