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Foreign reserves were supported by portfolio inflows, central bank deputy governor Perry Warjiyo said.
The rupiah strengthened 0.37 percent during October against the dollar and closed at 12,080 per dollar.
The rupiah traded at 12,170 against the dollar on Friday.
Warjiyo expects even stronger inflows after the government decides on when it will raise fuel prices, seen as crucial to reducing the country's budget and current account deficits.