  Radiation of supersoft X-ray sources (SSS) dominates both the supersof X-ray
and the far-UV domain. A fraction of their radiation can be reprocessed into
the thermal nebular emission, seen in the spectrum from the near-UV to longer
wavelengths. In the case of symbiotic X-ray binaries (SyXBs) a strong
contribution from their cool giants is indicated in the optical/near-IR. In
this paper I introduce a method of multiwavelength modelling the spectral
energy distribution (SED) of SSSs from the supersoft X-rays to the near-IR with
the aim to determine the physical parameters of their composite spectra. The
method is demonstrated on two extragalactic SSSs, the SyXB RX J0059.1-7505 (LIN
358) in the Small Magellanic Cloud (SMC), RX J0439.8-6809 in the Large
Magellanic Cloud (LMC) and two Galactic SSSs, the classical nova RX
J2030.5+5237 (V1974 Cyg) during its supersoft phase and the classical symbiotic
star RX J1601.6+6648 (AG Dra) during its quiescent phase. The multiwavelength
approach overcomes the problem of the mutual dependence between the
temperature, luminosity and amount of absorption, which appears when only the
X-ray data are fitted.Thus, the method provides an unambiguous solution. It was
found that selection of the model (a blackbody or an atmospheric model) is not
of crucial importance in fitting the global X-ray/IR SED. The multiwavelength
modelling of the SED of SSSs is essential in determining their physical
parameters.
