THE first student union of the Hongkong University of Science and Technology (HKUST) has finally been set up though not all positions have been filled. Its establishment early last month was a year behind schedule because of the failure to motivate enough students to cast their votes in order to ratify the constitution. It should have been passed in the 1991-92 academic year, the first year of the university's operation, but it failed to meet the quota of of votes. The constitution was not passed until last November when over half the student population cast their votes in support. Sean Lin King-sing, president of the newly-elected union, was a little disappointed about the lukewarm response. ''It seems that only a few students are interested in joining or care about union activity. This, unfortunately, is a common problem in other tertiary institutions as well. ''In fact, our situation is much better because we at least have a union finally; some colleges have no unions at all,'' he said. The University of Hongkong and City Polytechnic do not have student unions this year because there were no takers for the president and vice-president's posts. At last month's HKUST election, nearly half of the 2,000 full-time undergraduate and post-graduate students cast their votes to support the only group to join the election. Although the university now has its union, it is on a smaller scale. Only eight of the 12 positions were filled, leaving the posts of publicity, publication, social and entertainment, and external general secretary empty. ''Since the university has only first-and second-year students, looking for more suitable people to join is not easy. Students experienced in organising activities are already busy with other interest clubs. ''But the union is still attractive because we need not defer our studies in order to concentrate on union work. The university believes we can manage both academic study and activities,'' Sean said. Since both the university and union were new, Sean said they would focus on promotion work to make both better known to the public. ''We hope to let the public know more about our academic standard and excellent environment rather than the recent controversy over cost overruns. ''We will hold a series of activities like open days and walkathons to bring students closer and to build their sense of belonging.'' Sean said the union would register with the Government directly instead of with the university, so that it could be more independent in running its activities. Mr Lee Siu-ming, the university's student affairs officer, was impressed by students' active participation. ''Nearly 50 interest clubs have been formed in the past 18 months, much earlier than the setting up of the union. ''We are creating history from nothing; it requires a lot of effort and diligence. But this stimulates rather than discourages students from organising activities,'' he added.