  Thermal instability driven by radiation pressure might be relevant for
intrinsically bright accreting sources. The most promising candidate where this
instability seems to be at work is one of the two known galactic superluminal
sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relevance,
this scenario has not yet been confirmed by proper time-dependent modelling.
Non-linear time-dependent calculations performed by Szuszkiewicz and Miller
(1998) show that thermally unstable discs undergo limit-cycle behaviour with
successive evacuation and refilling of the central parts of the disc. This
evolution is very similar to the one proposed by Belloni et al. (1997) in their
phenomenological model. Further investigations are needed to confirm the
thermal instability being operational in this source. First of all the spectra
emitted from the disc during its evolution should be calculated and compared
with observations. Here such spectra are computed assuming local blackbody
emission from the best studied transonic disc model.
