Isotopic anomalies of platinum in the mercury-manganese star HR 7775

High-resolution spectra have been obtained for the regions of the five stro ngest optical lines of Pt rr in the spectrum of the cool HgMn star HR 7775, which is one of the sharpest-lined HgMn stars known. Model lines have been constructed from the isotopic and hyperfine structure laboratory analysis of Engleman. Abundances of the individual isotopes have been determined fro m spectrum synthesis. The total abundance of Pt is 4.46 dex greater than th e adopted solar abundance. The isotopic composition is clearly nonterrestri al, with a pronounced relative enhancement of the heaviest isotope,Pt-198, and deficiencies of isotopes lighter than Pt-196. Th, pattern of isotopic c omposition does not follow the widely assumed fractionation formalism; the lighter isotopes are far more deficient than a single-parameter fractionati on pattern would predict.