Illustration of the normalization method. Top panel: the flux calibrated observed spectrum of B243 in black. The photospheric model spectrum is indicated by the orange dashed line and the continuum of this model by the green dash-dotted line. The model spectrum is matched to the observed spectrum using a least squares method in the unhatched wavelength ranges. At the top the location of the Paschen lines and a DIB are indicated. Bottom panel: the photosphere corrected and normalized observed spectrum (in solid black) and photospheric model (in dashed blue). The horizontal blue line indicates unity.