We examine the relationship between superoutburst duration $t_{rm dur}$ and orbital period $P_{rm orb}$ in AM CVn ultra-compact binary systems. We show that the previously determined steep relation derived by Levitan et al (2015) was strongly influenced by the inclusion of upper limits for systems with a relatively long orbital period in their fit. Excluding the upper limit values and including $t_{rm dur}$ values for three systems at long $P_{rm orb}$ which were not considered previously, then $d log (t_{rm dur})/ d log (P_{rm orb})$ is flat as predicted by Cannizzo & Nelemans(2015)
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