Lithium-rich Cepheid V470 Cas

Aims. In this Letter, we report the discovery of a new lithium-rich yellow supergiant star – the Cepheid V470 Cas – that has a high lithium abundance, log A(Li) = 3.29. This is highly unusual for supergiant stars. V470 Cas is joining a very select group of lithium-rich Cepheids, with only nine members known to date in our Galaxy. Methods. For the analysis of our high-resolution echelle spectrum obtained at the Canada-France-Hawaii Telescope, methods based on both local thermodynamic equilibrium (LTE) and non-local thermodynamic equilibrium (NLTE) assumptions were applied. In particular, the lithium abundance was derived by analysing the equivalent width of the 6707 Å line. Results. Most lithium-rich Cepheids are located near the blue edge of the instability strip near the bottom of the Hertzsprung-Russell diagram. Their main sequence progenitors are distributed over a mass range of three to five solar masses. Of the nine lithium-rich Cepheids known, six are double-mode pulsators, including our programme Cepheid. It is very likely that the stars of this small group are entering the instability strip for the first time and have not yet passed the red giant phase. Therefore, they have not experienced the large-scale mixing event that could destroy lithium in their convective zones.