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Classical novae show a rapid rise in optical brightness over a few hours.Until recently the rise phase, particularly the phenomenon of a pre-maximumhalt, was observed sporadically. Solar observation satellites observing CoronalMass Ejections enable us to observe the pre-maximum phase in unprecedentedtemporal resolution. We present observations of V5589 Sgr with STEREO HI-1B ata cadence of 40 min, the highest to date. We temporally resolve a pre-maximumhalt for the first time, with two examples each rising over 40 min thendeclining within 80 min. Comparison with a grid of outburst models suggeststhis double peak, and the overall rise timescale, are consistent with a whitedwarf mass, central temperature and accretion rate close to 1.0 solar mass,5x10^7 K and 10^-10 solar masses per year respectively. The modelling formallypredicts mass loss onset at JD 2456038.2391+/-0.0139, 12 hrs before opticalmaximum. The model assumes a main-sequence donor. Observational evidence is fora subgiant companion; meaning the accretion rate is under-estimated.Post-maximum we see erratic variations commonly associated with much slowernovae. Estimating the decline rate difficult, but we place the time to declinetwo magnitudes as 2.1 < t_2(days) < 3.9 making V5589 Sgr a "very fast" nova.The brightest point defines "day 0" as JD 2456038.8224+/-0.0139, although atthis high cadence the meaning of the observed maximum becomes difficult todefine. We suggest that such erratic variability normally goes undetected infaster novae due to the low cadence of typical observations; implying erraticbehaviour is not necessarily related to the rate of decline.

Temporal resolution of a pre-maximum halt in a Classical Nova: V5589 Sgr observed with STEREO HI-1B