I propose that the properties of the two outbursts observed in the X-ray
transient XTE J1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In
these systems a `normal' outburst immediately precedes a 5-10 times longer
(`super')outburst. The optical light curve of the outbursts of XTE J1118+480 is
remarkably similar to that seen in some SU UMa stars, such as UV Per and T Leo,
where the precursor outburst is distinct from the superoutburst, but the time
scales are a factor of ~15 different. The first outburst of XTE J1118+480 was
relatively short (~1 month) while the second outburst was ~5 times longer.
During the second outburst superhumps were seen, a feature characteristic for
superoutbursts. The gap of about a month between the two outbursts is longer in
X-rays with respect to the optical, a feature not previously recognized for
X-ray transients. Also in SU UMa stars the precursor outburst becomes more
distinct at shorter wavelengths. Finally, I show that the time of appearance of
the superhumps in XTE J1118+480 is consistent with the expected superhump
growth time, if the superhump mechanism was triggered during the first
outburst. I conclude that the similarity in outburst behaviour in the two types
of systems provides further support that a common mechanism is at work to start
the long (`super')outbursts.Comment: 6 pages, 2 figures, to be published in Astronomische Nachrichten;
correct inclusion of figure