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 transient SAX J1808.4 - 3658 during 1998 April—June. The optical counterpart has been confirmed by several pieces of evidence. The optical flux shows a modulation on several nights that is consistent with the established X-ray binary 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 April 18 declined rapidly after April 22. From May 2 onwards the magnitude was more constant at around V=18.45 but by June 27 it was below our sensitivity limit. The optical decline precedes the rapid second phase of the X-ray decrease by 3 ± 1 d. The source has been identified on a 1974 UK Schmidt plate at an estimated magnitude of ̃ 20. The nature of the optical companion is discussed.