Based on observations obtained at the European Southern Observatory (ESO), La Silla, observing programmes 68.C-0536 and 70.C-0533, and on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

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Available for download at the TIMMI2 website (http://www.ls.eso.org/lasilla/sciops/timmi/docs/tables/).

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... strength'')

To estimate the continuum we simply interpolate linearly between 8 and 13 micron. The peak/continuum ratio is the maximum value of the normalized spectrum $F_{\rm norm}=1+F_{\nu,cs}/\langle F_{\nu,c}\rangle$ , where $F_{\nu,cs}$ is the continuum subtracted spectrum ( $F_{\nu}-F_{\nu,c}$ ) and $\langle F_{\nu,c}\rangle$ is the mean of the continuum. This definition of $F_{\rm norm}$ preserves the shape of the emission band even if the continuum is not constant. For a constant continuum level it is identical to $F_{\nu}/F_{\nu,c}$ .

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