Most HPF US magnetic ballasts (a vanishing breed) are the rapid-start or instant-start type. They generally have a capacitor in series with the lamp(s) to form some of the ballasting impedance as part of the design. There are some older preheat HPF ballasts that have a capacitor across the mains, but the most common of those, the "Tulamp" design, had a cap in series with one lamp instead.
Matt
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