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Used my Luxus U on tour in Ireland for a month. Rained a fair bit, and it was left outside overnight to get wet at least half the time. It was always fine. If you were really paranoid you could gaffer tape the button, as well as the charger wire (and it's adaptor) that comes out the button, which I did when it rained for long periods. I did have a pdw mudguard which stopped water getting up the bottom of the light unit, which might be a concern. Could always gaffer that bit too I guess.
For a start @hippy only needs to make sure that he can trust the Luxos in challenging weather.