Sanctions are once again catching up on the ruble. While still the best performer in the world this year, no emerging-market currency has fallen more in the past month as the effect of rebounding oil prices faded. Approaching foreign-debt payments and a shrinking economy probably mean it has a further 10 percent to drop this quarter, according to BNP Paribas SA’s Piotr Chwiejczak, the most accurate ruble forecaster in the four quarters through June 30. After a year of sanctions over Ukraine, Russian corporations remain shut out of foreign capital markets, meaning someone needs to sell rubles every time an international debt is paid. As well as a corporate bill this quarter that’s more than half as big again as the previous three months, the ruble is also under pressure as the Bank of Russia continues to buy foreign exchange, fueling speculation the government is demanding a weaker currency. […]