  Context. We report the discovery of a new luminous supersoft source, XMMU
J005455.0-374117, in the nearby spiral galaxy NGC 300, in XMM-Newton
observations performed on 2005 May 22 and on 2005 November 25. The source is
not present in ROSAT data nor in the previous XMM-Newton observations of 2000
December/2001 January. The unique luminous supersoft source, XMMU
J005510.7-373855, detected in the 1992 May/June ROSAT data and in the 2000/2001
XMM-Newton data, fell below detectability. This source already appeared highly
variable in ROSAT observations. Aims. We report on the temporal and spectral
analysis of this new supersoft source (SSS) and compare its properties with the
previous known SSS. Methods. We present the light curves of the SSS, model its
spectrum and estimate the corresponding flux and luminosities. Results. The
light curve of XMMU J005455.0-374117 does not show large fluctuations in any of
the observations and its spectrum can be modelled with an absorbed blackbody
with kT~60eV. The corresponding bolometric luminosity is 8.1e38 erg/s in the
first observation and drops to 2.2e38 erg/s six months later. No optical source
brighter than mV~21.7 mag is found coincident with its position. Conclusions.
The luminosity of these two SSSs is higher than what has been found for
`classical' SSSs. Their nature could be explained by beamed emission from
steady nuclear burning of hydrogen onto white dwarfs, or accretion onto
stellar-mass black hole with matter outflow or observed at high inclination
angle. The presence of an intermediate-mass black hole seems unlikely in our
case.
