  The detection and study of molecular gas in born-again stars would be of
great importance to understand their composition and chemical evolution. In
addition, the molecular emission would be an invaluable tool to explore the
physical conditions, kinematics and formation of asymmetric structures in the
circumstellar envelopes of these evolved stars. However, until now, all
attempts to detect molecular emission from the cool material around born-again
stars have failed. We carried out observations using the APEX and IRAM 30m
telescopes to search for molecular emission toward four well studied born-again
stars, V4334 Sgr, V605 Aql, A30 and A78, that are thought to represent an
evolutionary sequence. We detected for the first time emission from HCN and
H$^{13}$CN molecules toward V4334 Sgr, and CO emission in V605 Aql. No
molecular emission was detected above the noise level toward A30 and A78. A
first estimate of the H$^{12}$CN/H$^{13}$CN abundance ratio in the
circumstellar environment of V4334 Sgr is $\approx$3, which is similar to the
value of the $^{12}$C/$^{13}$C ratio measured from other observations. We
derived a rotational temperature of $T_{\rm rot}$=13$\pm1$ K, and a total
column density of $N_{{\rm HCN}}$=1.6$\pm0.1\times$10$^{16}$ cm$^{-2}$ for
V4334 Sgr. This result sets a lower limit on the amount of hydrogen that was
ejected into the wind during the born-again event of this source. For V605 Aql,
we obtained a lower limit for the integrated line intensities $I_{^{12}\rm
C}$/$I_{^{13}\rm C}$>4.
