An unresolved problem regarding ancient astronomical records is that of the star `Red Sirius'. While Sirius today shines white with a blueish hue quite in agreement with its spectral type AIV, many Greek/Roman and Babylonian sources (although still disputed) definitely assign a red colour to this star. We now present new and apparently independent information about Red Sirius from an early-medieval manuscript. This manuscript is of Lombardic origin (8th century) and contains the otherwise lost `De cursu stellarum ratio' by Gregory of Tours (about AD 538-593). It is preserved in the library of Bamberg1. Red stars in ancient records are those with colour index B-V = 1.0 or greater. Assuming an unchanged Sirius A, this lower limit for the combined colour of Sirius A and B allows the computation of the region of pre-white dwarf Sirius B in the Hertzsprung-Russell diagram or colour-magnitude diagram (Figs 1, 2). Sirius B lies on the giant branch, which fits well with our observational and theoretical framework of stellar evolution. However, the timescale of transformation of a red giant to a white dwarf is surprisingly short.