1 Introduction

For the astronomical community analysing ISO-SWS data (Infrared Space Observatory, Short-Wavelength Spectrometer, de Graauw et al. 1996), a first point to assess when judging and qualifying their data concerns the flux calibration accuracy. Since the calibration process is not straightforward, knowledge on the full calibration process and on the still remaining calibration problems is crucial when processing the data.

One way to detect calibration problems is by comparing observed data with theoretical predictions of a whole sample of standard calibration sources. But, as explained in Decin et al. (2000) (hereafter referred to as Paper I) a full exploitation of the ISO-SWS data may only result from an iterative process in which both new theoretical developments on the computation of stellar spectra - based on the MARCS and TURBOSPECTRUM code (Gustafsson et al. 1975; Plez et al. 1992; Plez et al. 1993), version May 1998 - and more accurate instrumental calibration are involved.

Precisely because this research entails an iterative process, one has to be extremely careful not to confuse technical detector problems with astrophysical issues. Therefore, the analysis in its entirety encloses several steps. Some steps have already been demonstrated in the case of $\alpha$ Tau in Paper I. They will be summarised in Sect. 2. Other points will be introduced in Sect. 2 and will be elaborated in the first sections of this article (Sects. 3-5). Having described the method of analysis, the general discrepancies between observed and synthetic spectra are subjected to a careful scrutiny in order to elucidate their origin. At this point, a distinction can be made between discrepancies typically for warm stars and those typical for cool stars. For this research, warm stars are defined as being hotter than the Sun ( $T_{{\rm eff},\odot} = 5770$ K) and their infrared spectra are mainly dominated by atomic lines, while molecular lines are characteristic of cool star spectra. A description on the general trends in the discrepancies for warm and cool stars will be made in this paper, while each star of the sample will be discussed individually in two forthcoming papers in which also an overview of other published stellar parameters will be given.

As stated in Paper I, the detailed spectroscopic analysis of the ISO-SWS data has till now been restricted to the wavelength region from 2.38 to 12 $\mu$m. So, if not specified, the wavelength range under research is limited to band 1 (2.38-4.08 $\mu$m) and band 2 (4.08-12.00 $\mu$m). Band 3 (12.00-29.00 $\mu$m) will be elaborated on by van Malderen (van Malderen et al. 2001, in prep.).

This paper is organised as follows: in Sect. 2 the general method of analysis is summarised. The sample of ISO-SWS observations is described in Sect. 3, while the data reduction procedure is discussed in Sect. 4. The observations of two independent instruments are introduced in Sect. 5. In Sect. 6, the results are elaborated on. In the last section, Sect. 7, the impact on the calibration of ISO-SWS and on the theoretical modelling is given.

The appendix of this article is published electronically. Most of the grey-scale plots in the printed version of the article are printed in colour in the appendix, in order to better distinguish the different spectra.