The first confirmed superoutburst of the dwarf nova GALEX J215818.5+241924

In 2011 October a possible nova was reported in Pegasus. The visible object had an ultraviolet counterpart, GALEX J215818.5+241924. We report unfiltered photometry of the object which revealed the presence of superhumps, with peak-to-peak amplitude of up to 0.22 magnitudes, diagnostic of it being a member of the SU UMa family of dwarf novae. The outburst amplitude was 4.6 magnitudes and it lasted at least 10 days, with a maximum brightness of magnitude 14.3. We determined the mean superhump period from our first 5 nights of observations as Psh= 0.06728(21)d. However analysis of the O-C residuals showed a dramatic evolution in Psh during the outburst. During the first part of the plateau phase the period increased with dPsh/dt= +2.67(15)×10^-4. There was then an abrupt change following which the period decreased with dPsh/dt= -2.08(9)×10^-4. We found a signal in the power spectrum of the photometry which we tentatively interpret as the orbital signal with Porb= 0.06606(35)d. Thus the superhump period excess was e= 0.020(8), such value being consistent with other SU UMa systems of similar orbital period.