A FORMER US Immigration and Naturalisation Service investigator pleaded not guilty in the District Court to six counts of graft. William Yeung Chung-wai, 41, will go on trial on February 21, next year. An additional charge of soliciting an advantage was preferred against him yesterday. The charge alleged that on a day between May 31 and July 1, last year, while an employee of the US Immigration and Naturalisation Service and without lawful authority, he solicited a gift or reward. It is alleged that he sought US$4,000 (HK$30,900) from Cheng Kin-ting as an inducement to him assisting Cheng on his application for a US work permit. The other counts relate to work permits.