The Act of Annexation was the Manticoran law under which the some star systems were joined to the Star Kingdom of Manticore.
Trevor's Star Edit
In case of Trevor's Star System the Act of Annexation specified that the House of Lords could not be increased by more than ten percent by San Martino peers between any two general elections and none could be seated before the first general election after the annexation. The High Ridge government refused to call a new general election, fearing that a change in the House of Lords would result in their loss of control of the government. (HH10)
The Act of Annexation was issued before the Republic of Haven ceded the Trevor's Star System to the Star Kingdom of Manticore. (HH12)