Four more officers from the Corrupt Practices Investigation Bureau (CPIB) took the stand on Monday as the sex-for-grades corruption trial of NUS law professor Tey Tsun Hang entered its eighth day.

Unlike the previous witnesses from the bureau, however, Tey had nothing to ask CPIB officers Michael Oh, Raymond Wee, Thomas Cheo and Hasvind Elangovan.

When given the opportunity to cross-examine each of them after their initial examination by the prosecution, Tey, who is conducting his own defence, declined, saying he was unable to remember them.

He claimed that he could not recognise all the officers called in the morning as the events had been "traumatising" and his "memory was sparse".

CPIB deputy director Teng Khee Fatt took the stand just before the court session broke for lunch and will return when the session resumes at 2.30pm.

Tey, 41, faces six charges of corruptly obtaining gifts and sex from former student Darinne Ko, 23, in exchange for better grades. These allegedly took place between May and July 2010.

The gifts included a $740 Montblanc pen, two tailored shirts worth $236.20, a $160 iPod touch, and a $1,278.60 payment for a dinner Tey hosted. The sex took place in his National University of Singapore (NUS) office on July 24 and 28, 2010.