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tokens: stage. From our observations of SDSS 1137 we also see that the binary mostly emits in blue bands as occurs in other disc accreting AM CVns. The X-ray emission is an order of magnitude smaller than the UV one, and the X-ray spectrum is also soft (consistent with the one of SDSS 1411 within errors). This suggests that the X-ray component is less important in AM CVns with discs than the UV component. The main uncertainty in the light curve of Fig. 1 is the duration of the outburst. This is due to the poor photometric coverage at early epochs (between 2017 December 21 and 2018 March 25), as well as the instrumental fluctuations and relatively large photometric uncertainties of the ZTF data, especially in the g band. However, an upper limit can be established considering the observation date of the XMM – Newton data (2017 December 21) and the beginning of the first rebrightening, which then imply an outburst duration of 380 d. For comparison purposes we have plotted that value in the outburst duration versus orbital period relation for AM CVns (Fig. 4 ). This duration is comparable to that of the recently reported value for SDSS 0807 (Rivera Sandoval et al. 2020 ), for which the first detected superoutburst lasted 390 d. It is clear that these two systems do not follow expectations from the DIM. They exceed by several times their expected duration, suggesting that there are additional mechanisms at work causing the outbursts of (long-period) AM CVn binaries. Figure 4. Orbital period ( P orb ) versus outburst duration (τ dur ) for AM CVns. The orange line indicates the relation obtained from the DIM (Cannizzo Ramsay 2019 ) and the blue line the empirical relation (Levitan et al. 2015 ) derived for systems with P orb up to 37 min. Upper limits are marked in cyan colour. We superimpose the results for SDSS 1137 from this paper, for SDSS 0807 (Rivera Sandoval et al. 2020 ) and for SDSS 1411 (Rivera Sandoval Maccarone 2019 ). The first two systems clearly exceed the expected τ dur from the DIM by several times suggesting the presence of additional (or even different) mechanisms in the origin of the outbursts, while SDSS 1411 still fits the predictions from the model. Note that the duration of the outburst in SDSS 1137 is an upper limit, where the yellow circle denotes the duration between the first ZTF measurement and the chosen date for the end of the outburst (See Section 4 ). The dashed line indicates the AM CVn with the longest P orb identified so far (SDSS J1505+0659, P orb = 67.8 min) as reported by Green et al. ( 2020 ) and which up to date has not shown outbursts. Other data in the plot were taken from Cannizzo Ramsay ( 2019 ) and references therein. A error for the outburst duration for all systems that do not have an upper limit was considered, as done by Levitan et al. ( 2015 ). 4.1 Colour evolution during outbursts of AM CVns: evidence of enhanced mass-transfer In order to investigate the mechanism(s) that drive outbursts in long-period