ASIAN central bankers are to join Hong Kong in pledging to use dollar-instrument repurchasing agreements to support each other's currencies against attack by speculators or a panic flight of hot money. The move will provide instant liquidity for a fast and concerted defence. It will be another weapon in Asia's anti-speculative armoury and a feather in the cap for the Hong Kong Monetary Authority's Joseph Yam. The authority fought off the run on the Hong Kong dollar in the wake of last winter's Mexican peso crisis with an admirably steady hand. Other emerging market currencies were also affected, but it was Hong Kong that took the lead in bringing the region's central banks into an alliance against speculators. The authority called a meeting immediately following the crisis to examine the region's defences and has been building relationships on several fronts to ensure co-operation and intelligence in case of future attacks. It has also, sensibly, joined the Southeast Asian Association of Central Banks and has now won the backing of other, similarly vulnerable countries, for its policies. A concerted surprise-attack against several markets by an alliance of well-funded speculators could still sap regional reserves or force wealthier systems to pour money down black holes in less well funded economies. But, nevertheless, joint action is a signal to hostile money-market players to act cautiously or risk losing their shirts. The warning is clear: stable currencies are too important to Asia to put them at risk through lack of preparation and failure to co-operate. There will be other Mexicos, but Hong Kong and its allies are prepared to ensure the subsequent turmoil on the stock and money markets does as little long-term damage here as the peso collapse did in January.