Announcing its second acquisition deal in less than a week, UST Corp. of Boston said Monday that it had agreed to buy Affiliated Community Bancorp of Waltham, Mass., for $259 million in stock.

Affiliated, a $1.1 billion-asset thrift, would bolster UST's market position in Middlesex County, a string of affluent suburban communities west of Boston that UST executives deem critical for expansion.

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