High resolution continuum imgaging observations of comets 4P/Faye and 19P/Borrelly with the Hubble Space Telescope have been analysed by means of a nucleus brightness extraction method. It is based on the fitting of a morphological model of the inner coma including the nucleus as an unresolved source to the image data. Nucleus sizes are deduced from the observations. Constraints on the shapes of both nuclei are derived from the nuclear light curve. Earlier preliminary results for 4P/Faye are revised and first results for 19P/Borrelly are presented. Additionally Afrho-parameters and dust production rates are reported for both comets.