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It is now well established that the photosphere of the cool HgMn star, $\chi$ Lupi, possesses overabundances of Pt (Z=78, [Pt II/H]=+4.1), Au (Z=79, [Au II/H]=+4.3), and Hg (Z=80, [Hg II/H]=+5.0). In each case only one isotope of the element is observed, the heaviest isotopes of Pt (198) and Hg (204), and Au (197), which is naturally monoisotopic. We announce here the detection of two hyperfine lines of the heaviest isotope of singly-ionized Tl (Z=81), mass number 205. New laboratory measurements of the spectrum of Tl, carried out at the University of Lund, provide wavelengths for the hyperfine components of both Tl (203) and Tl (205) for the 6s$^2$ $^1$S$_0$ - 6s6p $3P_1$$^o$ $\lambda$1908 \AA\ raie ultime transition, which are accurate to $\pm$0.0003 \AA. The isotopic components are separated by 0.011 \AA\ while the hyperfine components are separated by 0.136 \AA. Oscillator strengths have been obtained through ab initio, multiconfiguration Dirac-Fock (MCDF) calculations to an estimated uncertainty of 20\%. We have established the absolute wavelength scale of the GHRS observations to an accuracy of $\pm$0.001 \AA, so that the uncertainty in the match to the Tl (205) wavelengths is very much less than the separation of the isotopic components. Attempts to synthesize the observed features with both isotopes blended in solar-system proportions (30\% 203, 70\% 205) yields a poor match to the observations. The best fit is indistinguishable from the case of pure Tl (205), and yields an abundance of log N(Tl II)=4.66 (on a scale where log N(H) = 12.00) and [Tl II/H]=+3.8$\pm$0.1. Diffusion models or other theoretical scenarios must be prepared to explain not only the enormous overabundances of these very heavy elements, but also the fact that only a single isotope (the heaviest isotope) is invariably present.
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