The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar

A set of CCD images have been obtained during the decline of the X-ray tran sient SAX J1808.4 - 3658 during 1998 April-June. The optical counterpart ha s been confirmed by several pieces of evidence. The optical flux shows a mo dulation on several nights that is consistent with the established X-ray bi nary orbit period of 2 h, This optical variability is roughly in antiphase with the weak X-ray modulation. The source mean magnitude of V = 16.7 on Ap ril 18 declined rapidly after April 22, From May 2 onwards the magnitude wa s more constant at around V = 18.45 but by June 27 it was below our sensiti vity Limit. The optical decline precedes the rapid second phase of the X-ra y decrease by 3 +/- 1 d. The source has been identified on a 1974 UK Schmid t plate at an estimated magnitude of similar to 20. The nature of the optic al companion is discussed.