INVESTORS are still holding on to their share certificates despite the introduction of a central clearing system with depository bank facilities. The Hongkong Securities Clearing Co has revealed - after seven months of operation - that the total amount of stock under CCASS custody was 10 per cent of all issued stock of listed companies in the territory. Clearing company chief executive Richard Heckinger said yesterday: ''The number of shares coming into the depository is a little slow. ''Obviously we see increasing numbers when the turnover is high, to ease settlement.'' On the brighter side, the central clearing system now processes 55 per cent of all market transactions, 70 per cent of the total value of daily business done on the exchange and 35 per cent of the number of shares changing hands. ''We have spoken to the National Securities Clearing Corp of New York about the amount obtained under custody at present. ''They suggested that it took them some time to encompass the market and still there were some stocks of which they still only had 40 per cent of the stock in issue under custody,'' said Mr Heckinger. The registration fee to enter the system was a disadvantage of using the facility, he said. ''I think the low level of take-up is normal and we are happy with the transaction, value and volume levels of business going through the system,'' he said. With all the Hang Seng Index constituents now in CCASS, the system is concentrating on bringing in the market's smaller capitalised stocks. The process of bringing all the stock market's securities into the system is not expected to be completed for at least three months. The cumulative total of settlement instruction transactions settled in CCASS amounted to 396,574, involving 31.3 billion shares with a total value of $477.3 billion. At the end of January 10.1 billion shares were under the clearing company's custody at the CCASS depository, totalling a market value of $95 billion. The clearing house magazine ClearTalk said in the latest issue: ''From June 24, 1992, when CCASS started its operation, to the end of January 1993, the system registered continuous growth in the number of daily transactions processed through the system, both in exchange trades and settlement instruction transactions.'' Mr Heckinger said: ''At the moment we are fine-tuning the system in the light of actual operating experience.'' A transaction checking function is the first planned enhancement. The maker-checker function will provide additional control and division of responsibilities in executing settlements. ''Maker-checker functions give added protection to participants and enable them to strengthen their internal controls by segregating input and verification functions in order to reduce exposure to risk,'' said Mr Heckinger. Further enhancements were planned, but these would only reach implementation after thorough testing and research, he said.