Fig. 1. CO J =2 1 and SiO v =1 J =2 1 emission toward µ Cep. All the spectra have been obtained in January 1994 except the bottom one which was taken in June 1995. From top to bottom the backends are : 1 MHz filterbank, autocorrelator with =320 kHz, autocorrelator with =40 kHz, 100 kHz filterbank and autocorrelator with =40 kHz. All the observations were obtained with the wobbling system. A degree 1 baseline has been removed for each spectrum. Note the important change in the SiO line profile between 1994 and 1995.