Abstract One of the most striking results of Ariel 5 has been the large number of transient sources which has been observed, including what is probably a new type which is relatively short-lived and one near the Galactic Centre which is also probably distinct from the others since it was also observed as a radio source. Two of the short-lived X -ray novae were particularly well observed. The light curves resemble those of some optical sources, and the evolution of the X-ray spectra has been followed. The most unexpected result found was the modulation of the X -ray emission at periods of a few minutes.