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We report on multi-wavelength observations during quiescence and of the firstdetected outburst of the ~60 min orbital period AM CVn SDSS J113732+405458.Using X-ray and UV observations we determined an upper limit duration of theevent of about one year. The amplitude of the outburst was remarkably small, ofaround one magnitude in r and 0.5 magnitudes in g. We have also investigatedthe color variations of SDSS J113732+405458 and other long period AM CVns inoutbursts and identified a track on the color-magnitude diagram that is notcompatible with the predictions of the disk instability model, suggesting thatsome outbursts in long period AM CVns are caused by enhanced mass-transfer. Toour knowledge, these are the first studies of the color evolution in AM CVns.During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of~3E29 erg/s in the 0.5-10 keV band. This indicates a very low accretion rate,in agreement with the disk instability model for long period systems. However,such a model predicts stable disks at somewhat long periods. The discovery ofthis system outburst, along with similarities to the long period system SDSSJ080710+485259 with a comparably long, weak outburst, indicates that theseenhanced mass-transfer events may be more common in long period AM CVns. Alarger sample would be needed to determine empirically at what period, if any,the disk instability stops functioning entirely. Finally, we identified aninfrared excess in the quiescence spectrum attributable to the donor. Thismakes SDSS J113732+405458 the second AM CVn to have a directly detected donor.

The outburst of a 60 min AM CVn reveals peculiar color evolution: implications for outbursts in long period double white dwarfs